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.