Realgar (arsenic sulfide) is a striking mineral with a ruby-red to orange-red color and an adamantine luster. It is a very soft mineral, only 1.5 to 2 on the Moh’s hardness scale; it can be scratched simply by ones fingernail. It has a very common association with Orpiment, combining red with yellow to form a very colorful and aesthetic pairing. In addition to Orpiment, other associations include Calcite, Quartz, Barite, Sphalerite, Stibnite and Pyrite. Though this colorful mineral contains a significant amount of arsenic, it need not be feared or disregarded as a collectable mineral; rather, one should simply wash their hands following handling (this should be a normal practice when handling any and all minerals and rocks).

The crystal system for Realgar is monoclinic, often forming prismatic striated crystals. Other forms include coarse to fine granular, druzy, encrusting, botryoidal and massive.

Realgar specimens are found in limited localities, including China, Peru, Romania, Switzerland, Turkey and the USA (Nevada, Washington).