Lapis Lazuli

Lapis Lazuli

General
Lapis Lazuli is a beautiful deep blue stone, although its exact colour varies because it is a mixture of minerals whose proportions vary. It has been known since ancient times, its name coming from a Persian word lazhward meaning blue, and its historical source being high in the Badakhshan mountains in north-eastern Afghanistan where it has been worked for over 6,000 years. It is opaque and slightly soft, therefore is almost always cut en cabochon, or as beads, rather than being facetted. It was also in ancient times sometimes called sapphirus, meaning blue. The word lapis is latin meaning stone.

Colour
Although lapis, as it is often called, is deep blue, its colour does show variation depending on its source and exact composition. Some material is lighter blue or greenish blue, some a rich purple blue, almost black. The best colour is an intense deep blue. The pigment ultramarine used to be made from crushed lapis lazuli, and probably provides the best description of its most desirable colour. It usually contains golden coloured flecks of pyrites. These are normally regarded as increasing the desirability, and are an indication of authenticity. Lapis also contains calcite, which sometimes shows as white flecks. These flecks are regarded as decreasing the beauty and desirability of lapis, especially if they are large or numerous, although tiny flecks can help to indicate that it is genuine.
The deep blue colour is mainly due to the presence of hauynite.

Sources
Apart from its traditional source in Afghanistan, lapis lazuli is also found in Chile, Siberia, Burma, and California.

Technical Information

Chemical Composition and Name	* See below
Hardness	5.5
Refractive Index	1.5 variable.
Specific Gravity	2.75 to 2.9

Notes
* Lapis lazuli is not a mineral but a rock which is complex aggregate (mixture) of other minerals. These minerals are:-
Hauynite (NaCa)₄ 8(S₃So₂)₁ 2(Al₆Si₆O₂₄).
Sodalite Na₈Cl₂(Al₆Si₆O₂₄)
Noselite Na₈(SO₂)(Al₆Si₆O₂₄)
Lazurite, which is an amorphous mixture of hauynite and sodalite.
According to Webster's Gems, these four minerals all crystallise in the cubic system, and are members of a group of rock-forming minerals known as feldpathoids, which are produced when the silica content of the host rock is insufficient to form completely true feldspar. Calcite is always present.