Mineral Guide > Precious Stones > Opal gem stone

Opal gem stone

"The Opal, when pure and uncut in its native rock," says Ruskin, in his lecture on Color, "presents the most lovely colors that can be seen in the world except those of clouds."

The opal is indeed one of the most fascinating of gems, yet often elusive and at times disappointing. Of its freaks and foibles strange stories are told. Gems of brilliant quality are known suddenly to have lost their hues never to regain them, while others previously dull and lusterless have become radiant as the rainbow.

Chemically, opal is oxide of silicon with varying amounts of water, the water varying from 3 to 9 per cent. It is, therefore, closely allied to quartz, but differs physically in being softer and not as heavy. Further, it never crystallizes, and it is soluble in caustic potash, which quartz is not. It is infusible, but cracks and becomes opaque before the blowpipe. In sulphuric acid it turns black, on account, probably, of the organic matter it contains.

Opal as a mineral is quite common, so that no one need suppose, because he has specimens labeled " opal " in his collection, that he has as many precious stones. It occurs in many varieties, and, especially if it contains foreign matter, in many colors. Nearly all silica deposited by hot waters is in the form of opal, so that the geysers of Yellowstone Park build up cones of opal and fall into opal basins. This particular form of opal is known as geyserite, and it is often differently colored by different ingredients.

Wood is often preserved by silica in the form of opal, the siliceous waters taking away the wood and replacing it by opal, grain by grain, with such delicacy and accuracy that the structure of the wood is perfectly maintained.

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