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.