Realgar: The beautiful, toxic mineral that turns into dust in the light

Nature · Published 3 October 2026 Swipe more like this →

In short: Known historically as ruby of arsenic, this vibrant orange-red mineral has been used for centuries as a pigment, medicine, and pesticide. Yet, its beauty is deceptive. Exposure to light triggers a chemical transformation, causing this crystalline structure to crumble into a yellow powder, revealing its volatile, arsenic-rich nature.

Realgar, an arsenic sulfide mineral with the chemical formula α-As4S4, is a striking orange-red substance that has captivated human history for millennia. Its name derives from the Arabic term for 'powder of the mine,' reflecting its long-standing extraction as a minor ore of arsenic. Geologically, it typically forms in low-temperature hydrothermal veins, volcanic sublimations, and hot spring deposits, often appearing alongside orpiment, calcite, and barite. While it can be found in regions like Hungary, Bohemia, and Saxony, notable deposits in the United States include locations in Utah, Nevada, and Yellowstone National Park.

The mineral's history is as dangerous as it is diverse. Ancient Greek apothecaries utilized it to create a lethal concoction known as 'bull's blood,' which was famously associated with the suicides of figures like Themistocles and Midas. Beyond its use as a poison for rats in medieval Spain and 16th-century England, realgar served as a vibrant red pigment in art across China, India, and Egypt, and even in Venetian Renaissance painting. In Chinese culture, it is known as 'masculine yellow'—a counterpart to the 'feminine yellow' of orpiment—and is traditionally infused into liquor to create Xionghuang wine.

The most peculiar characteristic of realgar is its instability. When subjected to prolonged exposure to light, it undergoes a phase change, disintegrating into a distinct yellow powder called pararealgar (β-As4S4). For a long time, this degradation product was mistakenly identified as orpiment. This sensitivity to light also limits its utility in modern applications, such as decorative carvings, which eventually deteriorate. Despite its toxicity, realgar has been employed in firework compositions to produce yellow smoke and white flames, and it remains a subject of study for its unique chemical properties and historical significance in global trade and traditional medicine.

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