Lapis Lazuli

Lapis lazuli is not a single mineral - it's a metamorphic rock composed primarily of lazurite (which provides the blue), calcite (white), and pyrite (metallic gold). It forms when limestone is subjected to contact metamorphism - heated and chemically altered by the intrusion of nearby magma.

During this process, the calcium carbonate of the limestone reacts with sulfur-bearing fluids from the magma, forming lazurite and other sulfur-containing minerals. The transformation happens at temperatures between 500-600 degrees Celsius, deep enough underground that pressure keeps the volatile sulfur compounds from escaping.

The finest lapis comes from the Sar-e-Sang mines in the Kokcha River valley of Afghanistan - the same source that supplied the ancient Egyptians, Mesopotamians, and Indus Valley civilizations. These mines sit at an elevation of over 2,500 meters in the Hindu Kush mountains, and their lapis is prized for its intense, uniform blue with minimal calcite and just enough pyrite to add golden sparkle.

Quality lapis lazuli has a deep, uniform blue color without excessive white calcite patches. The scattered flecks of gold-colored pyrite are considered desirable. They resemble stars in a night sky. A blue streak on unglazed porcelain confirms the presence of lazurite.

Lapis is easily distinguished from sodalite (which it resembles but which lacks pyrite and tends toward a more violet-blue). It's also softer than many blue gemstones. At 5.5 Mohs, it can be scratched by a steel knife. The calcite component will effervesce (fizz) if you apply a drop of hydrochloric acid to an inconspicuous spot, which confirms the rock identity.