Realgar

Realgar is an arsenic sulfide mineral (As₄S₄), prized for its vivid crimson-red color and one of the softest gem materials known. Realgar forms in low-temperature hydrothermal veins, hot spring deposits, and as a volcanic sublimate. It often occurs alongside its yellow cousin orpiment (As₂S₃), with which it shares many geological environments. Both are arsenic sulfide minerals.

Fine crystals from China's Hunan Province reach several centimeters and display a stunning crimson color with resinous to adamantine luster. These form in hydrothermal veins within carbonate host rocks where arsenic-rich fluids deposited their mineral load as temperatures dropped.

Realgar is light-sensitive. Prolonged exposure to sunlight causes it to slowly decompose into paraealgar, a yellow powdery mineral, through a light-induced structural rearrangement. Museum specimens must be stored in darkness to preserve their vivid red color. This instability is why ancient realgar pigments in artwork have often turned yellow or orange over centuries.

Realgar is identified by its vivid crimson-red color, very low hardness (1.5, easily scratched), and orange-red streak. The resinous luster and monoclinic crystal habit (short prismatic crystals) are additional features.

Distinguish from cinnabar (higher SG, trigonal, red streak), crocoite (lead chromate, different streak), and red glass. Realgar's extreme softness and light sensitivity are key identifiers. SAFETY NOTE: Realgar contains arsenic. Wash hands after handling and don't store with other minerals.