Emerald

The name emerald derives from Greek smaragdos. It means "green stone" and, in ancient times,  referred not only to emeralds but also probably to most green stones.

Emerald is the most precious stone in the beryl group. Its green is incomparable, and is therefore called "emerald green." The coloring agent for the "real emerald" is chrome. Beryls that are colored by vanadium ought to be called​​​​ "green beryl" and not emerald. The color is very stable against light and heat, and alters at 700-800 degree C. The color distribution is often irregular; a dark slightly bluish green is most desired.

Only the finest specimens are transparent. Often the emerald is clouded by inclusions. These are not necessarily classified as faults, but are evidence as to the genuineness  of the stone as compare with with synthetic and other imitations. The expert refers to these inclusion as jardin ( French -Garden ).
The physical property, especially the density, refractive index, and double refraction, as well as the pleochroism, vary according to source area. All emeralds are brittle and combined with internal stress, sensitive to pressure; care must be taken in heating the. They are resistant to all chemicals which are normally used in the household.

Deposits Emeralds are formed by hydrothermal processes associated with magma and also by metamorphism. Deposit are found in biotire schists, clayshales, in lime-stones, wtih pigmatites.

Mining is nearly exclusively from host rock, where the emerald has grown into small veins or on wall of cavities. Alluvial placer are very unlikely to come about as the density of emerald is near that of quartz. Therefore, rare secondary deposits are mostly formed by weathering.

Significant deposits are in Colombia, especial​​y the Muzo mine northwest of Bogota. First mined by native tribes, the Muzo deposit was abandoned and rediscovered in the 17th century. The mine yields fine quality stones of a deep green color. Mining apart from shafts , is mainly by step form terraces. The emerald bearing, soft broken rock is loosened with sticks, lately also through blasting or with bulldozers, and the emeralds picked out by hand. the host rock is a dark carbonaceous limestone. Accompanying minerals are albite, apartite, arangonite, barite, calcite, dolomite, fluorite, and pyrite.

Another important Colombian deposit, the Chivor Mine, is northeast of Bogota. It was also mined by Native Americans. The host rock is gray-black shale and gray limestone. It is mined in terrace, and also from shafts.

During recent decades further emerald deposits, which promise to be successful, have been found in Columbia. There is always a high demand for the rare, so called Trapiche emerald found exclusively in Colombia, a wheel like growth of several prismatic crystals. Only a third of the Colombian emeralds are worth cutting Stones larger than nut size are usually low quality or broken.

In Brazil there are various deposit in Bahia, Goias, and Minas Gerais. Stone are lighter than Colobian ones, mostly yellow green, but they are often free of inclusions. Through deposits newly discovered since the beginning of the 1980s, Brazil has become one of the most important suppliers of emeralds.

Since the second half of the 1950s, emerald deposits have been explored in Zimbabwe. Most important is the Sandawana mine in the south. Crystals are small, but of very good quality.

In the northern Transvaal (South Africa ), emeralds are mined by modern methods using machinery ( Cobra and Somerset Mines ). Only five percent of the production is of good quality. Most stone are light or turbid and only suitable for cabochons.

Emerald deposits were discovered in 1830 in Russia in the Urals north of Yeka

Aquamarine

Aquamarine ( Latin-water of the sea) is so named because of its seawater color. A dark blue is the most desired color. The coloring agent is iron. Lower qualities are heated to 400-450 degrees C to change them to the desired, permanent aquamarine blue. Higher heat will lead to discoloration. Care must be taken when making jewelry Color can also be improved with neutron and gamma irradiation, but these changes do not last. Aquamarine is brittle and sensitive to pressure. Inclusions of fine, oriented hollow rods or aligned foreign minerals rarely cause a cat's-eye effect or aster-ism with six-ray stars with a vivid sheen.

Deposits The most important deposits are in Brazil, spread throughout the country. The well-known deposits in Russia ( the Urals ) seem to be worked out. Other deposits of some commercial significance are in Australia ( Queensland ), Burma ( Myanmar ), China,  India, Kenya, Madagascar, Mozambique, Namibia, Nigeria, Zambia, Zimbabwe, and the United States. The host rock is pegmatite  and coarse grained granite as well as their weathered material.
The largest aquamarine of gemstone quality was found in 1910 in Marambaya, Minas Gerais (Brazil). It weighed 243 lb ( 110.5kg ), was 18 in ( 48.5 cm ) long and 15.5 in ( 42 cm ) in diameter, and was cut into many stones with a total weight of over 100,000 ct. There have been finds weighing a few tones, but these aquamarines are opaque and gray, not suitable for cutting.
The preferred cuts are step ( emerald ) and brilliant-cut with rectangular or long oval shapes. Turbid stones are cut en cabochon ore are used for necklace beads.

Possibilities for Confusion with eudase, kyanite, topaz, tourmaline, zircon, and glass imitation. Synthetic aquamarine can be produced but is uneconomical. The "synthetic aquamarine" sold in the trade is really aquamarine-colored synthetic spinel.

Famous Sapphire and Possibilities for Confusion

Large sapphires are rare. They are sometime named in the same way as famous diamonds. The American Museum of Natural History ( New York) owns the "Star of India" perhaps the largest cut star sapphire (536ct); also the "Midnight Star" a black star sapphire (116ct). The "Star of Asia" a star sapphire weighing 330ct, is owned by the Smithsonian Institution in Washington D.C.
Two famous sapphires (St. Edward's and the Stuart sapphire) are part of the Lincoln, and Eisenhower have been carved out of three large sapphire, each weighing roughly 2000ct.

Sapphire can be confused with various stones. The blue sapphire looks similar to benitoite, indicolite, iolite, kyanite, spinel, tanzanite, topaz, and zircon, as well as blue glass.
Some imitation are made from doublets-blue cobalt glass with a crown of garnet or crown of green sapphire and a pavilion of synthetic blue sapphire. Lately, doublets have appeared using two small natural sapphire. Star sapphire is imitated by using star rose quartz with blue enamel on a flat back; alternatively the star is engraved on the flat back of a synthetic cabochon or on glass.
Synthetic sapphire was produced with properties identical to the natural stone at the beginning of this century. Since 1947 synthetic star sapphire of gem quality have been sold.

December Gemstones

December-Turquoise, blue topaz and tanzanite represent a happy life that is filled with good fortune. Headaches may be relieved or prevented. Ancient practitioners believed that turquoise would actually change color when the wearer was ill or in danger.

The history of Turquoise.

What exactly are Turquoise? The word Turquoise is derived from the Greek word "Turkois" meaning "Turkish" because it was first brought from Turkey. Turquoise is formed over millions of years by a chemical reaction that occurs when water leaks through rocks which contain specific minerals such as copper and aluminium. The percentage of the minerals in the rock dictate the shade of the turquoise.

Turquoise meaning.

 Is regarded as a love charm. It is also a symbol of good fortune and success, and it is believed to relax the mind and to protect its wearer from harm. Turquoise rings, in particular, are thought to keep away evil spirits.

The history of Blue Topaz.

The word “topaz” is believed to derive from the Sanskrit word “tapas,” which can mean fire, heat, and passion. Others believe “topaz” comes from the Greek word “topazos,” which means “to search.” In 1969, Texas named blue topaz as its official state gem after miners discovered a small deposit of natural blue topaz underground.

Blue Topaz meaning.

Of all the precious stones in the world, blue topaz is the perfect fit for intellectuals. Writers, artists, and scholars give blue topaz meaning by wearing it because it is indeed made for them, promoting higher thinking and cognition. Other feelings attached to blue topaz are serenity, friendship, fidelity, integrity, and gentleness. Just looking into its calm, cool shade of sparkling blue can make you breathe a sigh of relief.

The history of Tanzanite.

Tanzanite is the blue/violet variety of the mineral zoisite (a calcium aluminium hydroxyl Sorosilicate) belonging to the epidote group. It was discovered in the Mererani Hills of Manyara Region in Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. Tanzanite is used as a gemstone, and naturally formed tanzanite is extremely rare, still found only in the Mererani Hills.

Tanzanite meaning.

Tanzanite is called a "stone of magic" that helps with spiritual awareness and psychic insight. It is also sometimes called the "workaholic's stone".  This is because it helps you slow down and take it easy. This in turn, brings success in more areas of life. It relieves stress and depression, and enhances composure, poise, and harmony. Tanzanite is also said to transmute negativity. Physically it is said in healing folklore and crystal healing to be beneficial for stress related illnesses and high blood pressure in particular.  Note that healing crystal meanings are spiritual supports to healing and are not prescriptions or healthcare information. Tanzanite is related primarily to the throat and third eye chakras.

Deposits of Sapphire

Host rock of sapphire are dolomotized lime stones, marble, basalt, or pegmatite. It is mined mainly from alluvial deposits or deposits formed by weathering, rarely from the primary rock. Production methods are usually very simple. The underground gem-bearing layer is worked from hand -dug holes and trenches. The separation of clay, sand, and gravel is done by washing out the gemstones due to their higher density. Final selection is made by hand. Sapphire is much more common than ruby, as the substances which lend color to sapphire are more in Australia, Myanmar, Sri Langka, and Thailand.
Australia deposits have been known since 1870. The host rock is basalt; the sapphire are washed our of the weathered debris. Quality is modest. Under artificial light, the deep blue stones appear inky, blue green, nearly black; lighter qualities have a green tint. In recent decade black star sapphires have been found in Queensland. Accompanying minerals are pyrope, quartz, topaz, tourmaline, and zircon. Since 1918 good blue qualities have been found in New South Wales.
The alluvial deposits in upper Myanmar near Mogok are partially worked with modern methods and yield rubies and spinels as well as sapphires. The host rock is pegmatite. In 1966 the largest star sapphire was found here, a crystal of 63,000 ct (281l/12.6kg ).

Sapphire have been found in Sri Langka since antiquity. The deposits are in the southwest of the island in the region of Ratnapura. The mother rock is dolomotize limestone, which is enclosed in granite gneiss. There are also 10-20-in (30-60 cm) thick river gravel placers ( called illam locally) that are exploited from a depth of 3-33 ft ( 1-10 m). Sapphires are usually light blue, with a tinge of violet. There are also yellow and orange ( Padparadscha ) varieties as well as green, pink, brown, and nearly colorless stones. also star sapphire. Accompanying minerals are apatite, epidote, garnet, quartz, ruby, spinel, topaz, tourmaline, and zircon.

There are two sapphire deposits in Thailand: one in the region of Chantaburi, southeast of Bangkok, the other one near Kanchantaburi, northwest of Bangkok. The host rock is marble and/or basalt; placers and deposits formed by weathering are mined. The stone are of good quality in various colors, including star sapphire. Blue sapphire have a deep color, but tend to have a tings of blue-green.

The most desired sapphire used to come from Kashmir ( India), where the deposits were situated at a height of 16,500 ft (500 m) in the Zaskar mountains. 

Production varied since 1880, and the deposits have apparently been worked out. The host rock is kaolin-rich pegmatite in crystalline schist. The decomposition product yields sapphire of deep cornflower-blue color, often with a silky sheen. Most stones sold today as Kashmir sapphire come from Myanmar.

In the late 1800s sapphire deposits were discovered in Montana( United State). The host rock is andesite dikes. Mining is carried out on the dike rock, also from weathered material. Color of sapphire varies and is often pale blue or steel-blue. Mining has been interrupted repeatedly since the end of the 1920s,  but has been steadier in recent years.

There are also significant sapphire deposit in Brazil, Cambodia, China, Kenya, Madagascar, Malawi, Nigeria, Pakistan, Rwanda, Tanzania, Vietnam, and Zimbabwe. Isolated star sapphire have found in Finland.

Sapphire

The name sapphire ( Greek-blue) used to be applied to various stones. In antiquity and as late as the Middle Ages, the name sapphire was understood to mean what is today described as lapis lazuli. Around 1800 it was recognized that sapphire and ruby are gem varieties of corundum. At first only the blue variety was call sapphire, and corundum of other colors ( with the exception of red) were given special, misleading names, such as "Oriental peridot" for the green variety and "Original topaz" for yellow type.

Today corundum of gemstone quality of colors except red are called sapphire. Red varieties are called rubies are call rubies. The various colorless sapphire qualified by description, e.g., green sapphire or yellow sapphire . Colorless sapphire is called leuko-sapphire ( Greek-white), pinkish orange sapphire Padparadsha (Sinhalese for ''Lotus Flower'').

There is no definite demarcation between ruby and sapphire. Light red, pink, or violet corundum are usually called sapphire, as in this way they have individual values in comparison with other colors. If they were grouped as rubies, they would be stones of inferior quality. The coloring agents in blue sapphire are iron and titanium; and in violet stones, vanadium. A small iron content results in yellow and green tones; chromium produces pink, iron, and vanadium orange tones. The most desired color is a pure cornflower-blue. In artificial incandescent light, some sapphires can appear to be ink-color or black -blue.

Through heat treatment at temperatures of about 3100-3300 degree F (1700-1800 degree C), some cloudy sapphire, non distinct in color, can change to a bright blue permanent color.
Hardness is the same as ruby and also differs clearly in different directions ( an important factor in cutting). There is no fluorescence for all sapphires.
Inclusion of rutile needles result in a silky shine; oriented, i.e., aligned, needles cause a six-ray star sapphire .

Famous Rubies

Ruby is one of the most expensive gems, large rubies being rarer than comparable diamonds. The largest cuttable ruby weighted 400ct; it was found in Burma and divided into three parts. Famous stones of exceptional beauty are the Ewardes ruby (167ct) in the British Museum of Natural History in London, the Rosser Reeves star ruby (138ct) in the Smithsonian Institution in Washinton, D.C., the De Long star (100ct) in the American Museum of Natural History in New York, and the Peace ruby (43ct), thus called because it was found in 1919 at the end of World War I.
Many rubies comprise important parts of royal insignia and other famous jewelry. The Bohemian St. Wenzel's crown (Prague), for instance, holds a non faceted ruby of about 250ct. But some gems , thought to be rubies, have been revealed as spiniels, such as the ''Black Princes's ruby'' in the English State Crown and the ''Timur Ruby'' in a necklace among the English crown jewels. The drop- shaped spinels in the crown of the Wittelsbachs dating from 1830 were also originally thought to be rubies.

                                                                    Black Princes's ruby
Today rubies were cut in the countries where they were found. Because the cutters usually aim for maximum weight, the proportions are not always satisfactory, so that many stones have to be recut by dealers in other countries. Transparent qualities are cut in step and brilliant cut; less transparent stones, en cabochon or they are formed to carving. Only synthetic rubies are used for watches and bearings, formerly the most important technical application for natural stones.
Possibility for confusion with almadite, pyrope, spinel, topaz, tourmaline, and zircon. Since the beginning of the 20th century, there have been synthetic rubies with gemstone quality; these resemble natural ones especially in their chemical, physical, and optical properties. But most of them can be recognized by their inclusions as well as by the fact that they. in contrast to natural rubies, transmit shortwave ultraviolet light.
Numerous imitations currently are on the market, especially glass imitations and doublets. These have a garnet crown and glass underneath. There are many false names in the trade such as Balas ruby (=spinel), Cape ruby (=pyrope), and Siberian ruby (= tourmaline).

Deposit of Ruby

The host rocks of ruby are metamorphic dolomite marbles, gneiss, and amphibolite. The yield of rubies from such primary deposits is not economically profitable. Rather, secondary alluvial deposits are worked. Because of its high density, ruby is normally separated through the washing of river gravels, sands, and soil, then concentrate, and finally picked out by hand.
Production methods are still as primitive as they were a hundred years ago in many location. In state-owned mines, on the other hand, the usage of machinery is not exactly the rule, but much more frequent than in private companies. Some state regulated companies (e-g., Mogok in Myanmar) lately even work with highly mechanized machinery both above-and underground.
                                     
                                                            Mogok in Myanmar

                                                                   Rubies from Burma

Some of the most important deposits are in Myanmar, Thailand, Sri Lanka, and Tanzania. For centuries, the most important have been in upper Myanmar near Mogok. The ruby-bearing layer runs several yards under the surface. Apparently pigeon's blood color. They are consider to be the most valuable rubies of all. Large stones are rare. Mineral found together with ruby, often also of gemstone quality, are precious beryl, chrysoberyl, garnet, moonstone, sapphire, spinel, topaz, tourmaline, and zircon. In the early 1990s large new deposits were discovered at Mong Hsu in Myanmar. Rubies from Thailand often have a brown or violet tint to them.They are found southeast of Bangkok in the district of Chantaburi in clayey gravels. Shafts are sunk to a depth of 20 ft (8 m). However, in recent years Thai ruby production has been declining.
In Sri Lanka deposits are situated in the southwest of the island in the district of Ratnapura. Rubies from these deposits( called illam by the local population) are usually light red to raspberry-red. Some of the rubies are recovered from the river sands and gravels.
Since the 1950s Tanzania has produced a decorative green rock, a zoisite(anyolite), with the most opaque rubies. Only a few crystals are cuttable, most being used as decorative stones. On the upper Umba River(northwest Tanzania), on the other hand, rubies with gemstone quality have been found that are violet to brown-red.
Other mining deposits are in Afghanistan, Australia (Queensland, New South Wale), Brazil, Indian, Cambodia, Kenya, Madagascar, Malawi, Nepal, Pakistan, Zimbabwe, Tajikistan, the Unite States ( Montana, North Carolina), and Vietnam.
Small ruby deposits can also be found in Switzerland (Tessin), in Norway, and the southwest Coast of Greenland.

                                                        Ruby's deposit in Cambodia

 

Ruby

Ruby is thus named because of its res color(Latin-ruber). It was not until about 1800 that ruby, as well as sapphire, was recognized as belonging to the corundum species. Before that date, red spinel and the red garnet were also designated as ruby.
The red color varies within each individual deposit, so it is not possible to determine the source area from the color. The designations ''Burma ruby'' and ''Siam ruby'' are therefore strictly erroneous, and refer more to quality that origin. The most desirable color is the so-called ''pigeon's blood,'' pure red with a hint of blue.
The distribution of color is often uneven, in stripes or spots. The substance that provides thee color is chromium, and in the case of brownish tones, iron is present as well. As a rough stone, ruby appears dull and greasy, but, when cut, the luster can approach that of diamond. Heat treatment is commonly used to improve the color.
Ruby is the hardest mineral after diamond. However, the hardness varies in different directions. Ruby has no cleavage, but has certain preferred directions of parting. Because of brittleness, care must be taken when cutting and setting.
Inclusion are common. They are not always indicative of lower quality, but show the different between a natural and a synthetic stone. The type of inclusion (minerals, growth structures, canals, or other cavities) often indicates the source area.
Included rutile needle bring about either soft sheen (call silk) or, when cut en cabochon , the rare cat's eye effect, or more often the very desirable asterism-a six -rayed star, which shimmers over the surface of the stone when it is moved. Nowadays there are also Trapiche Rubies on the market. Their appearance is equal to the Trapiche Emeralds.

        Natural Ruby from Cambodia

Valuation of Diamonds

All diamonds that De Beers brings on the market are classified beforehand into one of the 16,000 different standards of quality, according to form, quality, color and size. Besides London, there are such sorting centers in Lucerne in Switzerland, Gaborone in Botswana, Windhoek in Namibia, And Kimberley in South Africa.
Formerly, 20 percent of all diamonds were considered suitable for jewelry, having been of ''Gemstone quality.'' The rest were sold to the industry as so-called industrial diamonds to be used for drilling crowns, milling machines, cutting wheels,etc. Since 1983, another 20 percent have been classified as''almost gemstone quality'' and are cut mainly in India. The smallest diamonds and diamonds of lesser quality can be worked and offered to buyers other than the strictly controlled diamond market.
In the valuation of faceted diamond, color, clarity, cut, and carat are taken into consideration. These four c's decide the value of a diamond.





Grading for Color Diamonds are found in all colors. Mostly they are yellowish. In the grading, these are evaluated together with the purely colorless diamonds. The rarer strong colors (green, red, blue, and yellow), the so-called fancy color, are valued individually and fetch collector's prices. Brown and black diamonds also occur.
Formerly, terms and definitions in grading for color were not uniform and often confusing, until an international agreement was reached to cover the so-called "yellow series.'' This was publish in 1970 as RAL 560 A5E. Since then, various institute of America (GIA), the International Diamond Council (IDC), and the Confederation International de la Bijouterie, Joaillerie, Orfevreie des Diamants, Perles, et Pierres (CIBJO). Today, the IDC regulation, written in English,  are accepted worldwide. In the United State, the GIA system is most commonly used.
The old grading terms, the ''Old Terms,'' should not be used anymore. In fact, however, they are still in usage in the gemstone trade.
Experts use standard sample collections for consistent, comparison color grading.
Grading for Clarity In Germany only the inner perfection is understood as ''clarity,'' while in the United States and in Scandinavia aspects of the quality of the outer finish are taken into consideration. Enclosed minerals, cleavages, and growth lines affect clarity; they are collectively called inclusions, but formerly were called ''flaws'' or ''carbon spots.'' Polished diamond without any inclusion under a 10 x loupe are considered ''flawless.'' Inclusions visible with larger are not taken into account for grading.
Grading for Cut To grad for cut, the type and shape of cut, proportion, and symmetry as well as outer marks are taken into consideration. In Germany the normal cut is the ''fine brilliant cut'' in the rest of Europe the ''Scandinavian-standard brilliant.'' In the United States, the only widely used cut grading system is that of the American Gem Society, based on the ''AGS Ideal Cut.''

Diamond Imitation

The fact that the diamond can be confused, in appearance, with many gemstones can lead to fraud, although not in the legitimate retail trade. A colorless diamond looks similar to rock crystal, precious beryl, cerussite, sapphire, scheelite, sphalerite, topaz, and zircon. Also many yellowish stones can look like diamond to the eye of the layman.
A part from these, there are various synthetic stones that are used to imitate diamonds. Especially strontium titanate ( fabulite ), YAG, GGG (galliant), linobate, and cubic ziconia (CZ, djevalite) must be mentioned. A well known diamond imitation made from glass is the so-called strass.
In 1970 the first gem-quality diamonds were synthesized, but these still do not compete with natural diamonds. At first, they served only scientific purposes since they are too small to be worn as jewelry. In 2003, however, synthetic diamonds up to 2.5 ct appeared on the market. In the trade one also finds natural diamonds, colored artificially by various irradiation treatment.
Diamond-doublets are made with: upper part-diamond; lower part-synthetic colorless sapphire, rock crystal, or glass. Other doublets have synthetic spinel as upper part, and strontium titanate underneath.

Diamond

The name diamond refer to its hardness (Greek-adamas, the unconquerable). There is nothing comparable to it in hardness . Its cutting resistance is 140 times greater than that of ruby and sapphire, the gemstones next in hardness after diamond. However, the hardness of a diamond is different in the individual crystal direction . This allows one to cut diamond with diamond and/or with diamond powder. Because of the perfect cleavage, care must be taken not to accidentally bang against an edge of a diamond, and also when setting it. Its very strong luster sometime enables the experienced eye to differentiate between a diamond and its imitations. Diamond is generally insensitive to chemical reactions. High temperatures , on the other hand, can induce etchings on the facets. Therefore, special care must be taken during soldering.
In the last fifty years, it has been recognized that there are various types of diamonds with different characteristics. Science differentiates between type la, Ib, Ila, and IIb. This is of little importance to the trade, but does assist the cutter. Due to the optical effects, the high hardness, and its rarity, the diamond is considered the king of gemstones. It has been used for adornment since ancient times.

Value of Gemstones

There are no firm prices regarding gemstones. They are subject to fluctuations just as with other products. In the end, supply and demand regulate the gemstone market. In addition, economic purchasing power and fashion trends can heavily influence the demand for certain gemstones.  There are great differences in pricing and purchase between diamond and the other so-called colored gemstones.
Diamond Prices With diamonds, the consumer can compare prices easily. Value determining elements, such as color, purity, and cut, are internationally defined.and can be controlled. Of course, there are profit margins and pursuit of profit. One can, however, avoid outright fraud by dealing with diamond wholesalers and established jewelers . Here a code of honor prevails like nowhere else in world trade. On the diamond stock exchanges, the wholesale diamond markets, business has been sealed from even the earliest times with a firm handshake and a person's word.
Color stone prices With colored stones, price often fluctuates greatly, even in the serious trade, since gem extraction sometimes depends on chance happenings that cannot be predicted. As always, beauty, rarity, and gemstone size determine price. However, in contrast with diamond trade, there are no controlling organization, no possible exertion of influence on honest trade. Furthermore, many small businesses are active with extraction and production. They often seek direct contact with the consumer, particularly tourists who are in the buying mood, and deceitfully exploit their ignorance.
Price Lists Numerous institutions such as diamond stock exchanges, gemological facilities, and professional journals publish current market prices and long term price tendencies. This pricing information is intended for trade and traders and not for the consumer who incurs substantial surcharges set by traders.
Price of stones are usually expressed"per carat" .The carat price often progressively increases with the size of the stone. For example, a one carat piece might coast $750, but a two carat piece is not necessarily worth twice the amount, or @1500, but perhaps $3000 or more.
Less valuable stones such as agate are sold by grams and kilograms, not carat.

what is gem/gemstone?

There is no generally accept definition for the term gem or gemstone, but they all have something special, something beautiful about them. Most gemstones are minerals(e.g., diamond), mineral aggregates( such as lapis lazuli), or rocks ( such as onyx marble ). some are organic formations ( e.g., amber), and other gem materials are of synthetic origin( e.g., YAG ).
There is no definite demarcation.line , and woods, coal, bones, glass, and metals are all used for ornamentation. Some examples include jet ( a form of coal ), ivory ( tusks of elephants as well as teeth of other large animals), moldavite ( a glassy after-product of the striking of a meteorite), and gold nugget ( more or less large gold -lumps). Even fossils are sometimes used as ornamental material.
For some gemstone the source of specialness and beauty is the color, an unusual optical phenomenon, or the shine that makes them stand out in comparison to other stones. For other stones it is the hardness or an interesting inclusion that makes them special. Rarity also plays a role in the classification as gemstone.
Since the valued characteristics usually come into effect only through cutting and polishing, gemstones are also normally considered to be the cut stones. Cutting and polishing means refinement of what might be an other wise insignificant raw material.
There are several hundred distinct types of gems and gem materials. The number of the of the variations is about double that. From time to time, new gemstones are discovered or varieties with gemstone quality are found in minerals which have been already known.
Harder stones are suitable for jewelry, whereas softer stones are often sought after by amateur collectors as well as serious lapidaries.

Gemstones

Gems have intrigued humans for at least 10,000 years. The first known, used for making jewelry, include amethyst, rock crystal, ember, garnet, jade, jasper, coral, lapis lazuli, pearl, serpentine, emerald, and turquoise. These stones were reserved for the wealthy, and served as status symbols. Rulers sealed documents with their jewel-encrusted seals. Such treasures can now be admired at many museums and treasure-vaults.
Today, gems are worn not so much to demonstrate wealth, but rather jewelry is bought increasingly for pleasure, in appreciation of its beauty.
Certainly, also today, when purchasing a gemstone, a certain love for a special stone is part of it. Formerly, when people were less scientifically knowledgeable, gems always had an aura of mystery, something almost spiritual. That's why they were worn as amulets and talismans. Up to the present day, gemstones have sometimes been used as remedies against illness. They could be used in three different ways: the mere presence of the stone was sufficient to effect a cure; the gem was placed on the afflicted part of the body; or the stone was powdered and eaten.
Presently, medical science worldwide is experiencing a revival of the ideas of the Middle Ages in the use of precious stones through the doctrines of the Esoteric. As a  capital investment, however, of all gemstone really only diamonds are suitable. In fact, these have proven to hold on to their value, despite the travails of war or depression in the economy.
The truly modern problem is the imitation of gemstones that have become more and more sophisticated. Although replicas of precious matter have always existed, it is only in our modern times that imitating has turned into a booming economic sector with an often disreputable background.