NATURAL SAPPHIRE

Description:

Sapphire is the non-red variety of corundum, the second hardest natural mineral known to mankind. Its beauty, magnificent colours, its transparency but also its resistance and permanence are characteristics which gemstone lovers and experts assign to sapphire. The word sapphire is probably Phoenician in origin, coming to English from the Ancient Greek through the Latin sapphirus, which refers to a "blue gem". The value of sapphires depends on their size, colour and transparency.

The corundum group consists of pure aluminium oxide, which a long time ago was caused to crystallise into beautiful and splendid gemstones by the pressure and heat in the depths of the ground. Crystals are found in metamorphosed rocks, silica-poor rocks, alkali igneous undersaturated rocks, contact aureoles in altered aluminous shales, aluminous xenoliths in high temperature plutonic and hypabyssal rocks, metamorphosed bauxite deposits, and as a detrital material in sediments.

Sapphires are well known among the general public as being blue, but even though the blue colour is by far the most popular colour for sapphire, orange-pink, golden, white, and even black have generated much interest in the gem trade. It is the impurities in the aluminium oxide crystal that give the color variations. Pure sapphire is transparent. Traces of iron and titanium give sapphires a blue colour. Oriented rutile crystal inclusions cause a six-pointed-star light effect (called asterism) to form the popular Star Sapphire. A version which shows an asterism is called a star sapphire. However, when natural sapphires show an asterism, the shape of the star is usually somewhat irregular and sometimes indistinct. Sapphires display a different colouring and satiation depending on the perspective. Bright daylight makes most sapphires shine more vividly than the somewhat muted artificial light.

The oldest Sapphire mines are situated in Sri Lanka, where gemstones were mined in ancient times. The expert recognises these sapphires from the luminosity and brilliance of their light to medium blue colour. A lot of blue sapphires come from Thailand and Australia. The other major deposits in Asia are the ones in Burma, Cambodia, and India. African sapphires come from Madagascar, Mozambique, Tanzania and Zimbabwe. In Russia, gem-quality sapphires occur in the Ural Mountains and Kola Peninsula.

Characteristics:

Chemistry: variety of Corundum, Al2O3

Hardness (Mohs scale): 9

Density: 3.99 – 4.10

Refractive index: 1.760 – 1.779

Crystal system: trigonal

Crystal habit: euhedral, small rounded crystals, alluvial crystals and grains

Cleavage: none

Fracture: conchoidal, uneven

Transparency: transparent to translucent to opaque

Luster: vitreous, dull