Lazurite

Lazurite is the deep-blue feldspathoid that gives the ornamental rock lapis lazuli its colour. It is a sodium-calcium aluminosilicate of the sodalite group that also carries sulphur, which is responsible for its intense ultramarine blue — the pigment of the same name was once made by grinding lapis.

Properties

Lazurite is isometric, relatively soft (5–5.5) and usually occurs massive rather than as clean crystals; in lapis it is intergrown with calcite and flecks of golden pyrite. The best colour is an even, saturated violet-blue with minimal calcite.

Occurrence

It forms by contact metamorphism of limestone. The classic source, mined for thousands of years, is Sar-e-Sang in Badakhshan, Afghanistan; other deposits occur near Lake Baikal in Russia and in the Chilean Andes. Lazurite is the key component of lapis lazuli.