have the highest refraction index, and those used for jewels are very transpare.... pearl natur

Diamonds for jewelry are graded on the basis of colour from blue-white to yellow. Production of synthetic stones is far greater than the supply of natural rubies. The (blue variety of corundum) is considered one of the most valuable of precious stones. A sapphire's colouring usually indicates its origin. Sapphires from Thailand are very similar in colour to those from Myanmar; those from Sri Lanka are of different shades but incline toward violet. Sapphires, like rubies, can be cut so that, in the light, a beautiful, luminous six-pointed star appears on the surface of the gem. The honey-yellow variety is the best known, but there are also pink, red, blue, and the less-used colourless stones. Among the less-important and less-rare stones, the is quite widely used in its three varieties: orange, blue, and colourless. Matrix jewelry is cut from a stone such as opal or turquoise and the surrounding natural material, or matrix. Pearls from the Persian Gulf are usually cream-colou! red; those from Australia are white with greenish or bluish shades; golden-brown pearls come from the Gulf of Panama; those from Mexico are black or reddish brown; pink pearls are from Sri Lanka; and those from Japan are cream-coloured or white with greenish tones. After huge quantities of cultivated pearls invaded the world market, interest in natural pearls underwent a considerable decrease. In addition to pearl, a number of other organic materials, including , coral, ivory, and jet, are considered gems. This material is usually branchlike and occurs in a variety of colours, of which the most sought after are rose red to red. pearl natur