Crystal and stones » Properties and composition of precious stones

Turquoise

precious stones — Wed, 16 Sep 2009 16:18:42 +0000

The turquoise is a pseudomorph.

In colour it is blue or greenish-blue, sometimes opaque, varying between that and feeble translucency, though it should be said that in all forms, even those considered opaque, a thin cutting of the stone appears almost transparent, so that the usual classing of it among the opaque stones must be [...]

Lapis-Lazuli

precious stones — Wed, 16 Sep 2009 16:16:15 +0000

The lapis-lazuli, sometimes called “azure stone,” is almost always blue, though often containing streaks of white and gold colour, the latter of which are due to the presence of minute specks or veins of iron pyrites, the former and colourless streaks being due to free lime, calcite, and other substances which have become more or [...]

Silicates

precious stones — Wed, 16 Sep 2009 16:14:00 +0000

The chief of these are the garnets, crystallising in the cubic system, and anhydrous.

The garnet is usually in the form of a rhombic dodecahedron, or as a trisoctahedron (called also sometimes an icosatetrahedron), or a mixture of the two, though the stones appear in other cubic forms.

In hardness they vary from [...]

Zircon

precious stones — Wed, 16 Sep 2009 16:08:31 +0000

Zircon appears to have been first discovered by Klaproth in 1789, in the form of an earth, and six years later he found that the stone hyacinth contained a similar substance, both having the formula, ZrSiO4, and both having as their colouring agent ferric oxide.

There are several methods of obtaining the metallic element, [...]

Tourmaline

precious stones — Wed, 16 Sep 2009 16:04:44 +0000

The tourmaline is a most complex substance; almost every stone obtained has a different composition, some varying but slightly, with mere traces of certain constituents which other stones possess in a perceptible degree.

Consequently, it is not possible to give the chemical formula, which might, and possibly would, be found but seldom, even in [...]

Emerald

precious stones — Wed, 16 Sep 2009 16:02:37 +0000

Considering the particular emerald which is a variety of beryl -although the name emerald in the trade is applied somewhat loosely to any stone which is of the same colour, or approaching the colour of the beryl variety -this emerald only differs chemically from the beryl, just described, in possessing an addition of oxide of [...]

Beryl

precious stones — Wed, 16 Sep 2009 15:59:43 +0000

The beryl is a compound of silicates of beryllia and alumina, with the formula 3BeOSiO2 + Al2O3,3SiO2, or 3BeO,Al2O3,6SiO2. It differs very little indeed from the emerald, with the exception of its colour.

In the ordinary varieties this is somewhat poor, being mostly blue, or a dirty or a greenish yellow; the better kinds, [...]

Topaz

precious stones — Wed, 16 Sep 2009 15:57:38 +0000

The name topaz is derived from the Greek topazos, which is the name of a small island situated in the Gulf of Arabia, from whence the Romans obtained a mineral which they called topazos and topazion, which mineral to-day is termed chrysolite.

The mineral topaz is found in Cornwall and in the British Isles [...]

Chrysoberyl

precious stones — Wed, 16 Sep 2009 15:54:00 +0000

There are certain stones and other minerals which, owing to their possession of numerous microscopically fine cavities, of a globular or tubular shape, have the appearance of “rays” or “stars,” and these are called “asteriated.”

Several of such stones have been discussed already in the last chapter, and in addition to these star-like rays, [...]

Sapphire

precious stones — Wed, 16 Sep 2009 15:48:37 +0000

The Sapphire is not so easy to imitate, as its hardness exceeds that of the ruby, and imitations containing its known constituents, or of glass, are invariably softer than the natural stone.

As before remarked, almost any form of corundum other than red is, broadly, called sapphire, but giving them their strictly correct designations, [...]