Although this common arsenic sulfide mineral occurs worldwide, cut gem-quality realgar is extremely rare. This fine, red stone is very fragile, difficult to cut, and nearly impossible to wear.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Monoclinic. Crystals prismatic, striated; compact, powdery.|
|Colors||Dark red, orange-red.|
|Luster||Resinous to greasy.|
|Cleavage||Good 1 direction.|
|Transparency||Translucent to transparent.|
|Pleochroism||Strong: colorless to pale yellow.|
|Optics||a = 2.538; β = 2.684; γ = 2.704. Biaxial (-), 2V: 40°.|
|Etymology||From the Arabic rahj-al-ghar, meaning “powder of the mine.”|
|Occurrence||Low-temperature hydrothermal vein deposits, especially with ores of lead and silver.|
With rich red to orange-red colors, realgar has a hardness of only 1.5 to 2. A penny could scratch it. Since it’s sectile (cuttable) with good cleavage, faceting presents challenges. Stones seldom occur with transparency. Furthermore, this mineral decomposes slowly in light and turns into yellow, even softer, pararealgar. To cap all this, realgar also contains arsenic. “Ruby arsenic” and “ruby sulfur,” older names for this mineral, allude to its color and noxious components. (Of course, rubies and realgars are distinct gem species).
Not surprisingly, although realgars make lovely gems, you’ll find very few of these cut. Those will most likely reside in gem collections.
This mineral is one of the principal sources of arsenic. It has a long history of use as a poison as well as a medicine in traditional Chinese and Indian Ayurvedic practices. From ancient times to the Renaissance, cultures from the Mediterranean region to China used realgar to produce red pigment. Tanners used it as a hair remover.
Red cinnabar gems can come close to realgars in appearance, hardness (2 to 2.5), and streak. However, they have a far greater specific gravity, higher birefringence, and different optic character. Their toxicity also differs. Cinnabar contains mercury, not arsenic.
Gem-quality sources for realgars include:
- United States: Boron, California; Getchell Mine and Manhattan, Nevada; Mercur, Utah; King County, Washington (fine crystals up to 2 inches long, some gemmy).
- China; Czech Republic; Germany; Japan; Peru; Romania; Slovakia; Switzerland.
Occasional fragments of Washington crystals will cut gems to about 3 carats.
A jeweler’s torch could easily reach the temperature at which realgar melts, 307° C (585° F). Use great caution when working with this gem. Always clean yourself properly and avoid inhaling dust or fumes when handling this mineral. For more safety recommendations, consult these guidelines.
Store any realgars in a dark container to avoid decomposition. Avoid storing them with other harder, more common gemstones to avoid scratches. Don’t put realgars in contact with water or steam. This can also cause a release of toxic fumes.