Realgar — The Toxic Arsenic Sulphide Collected for Its Colour
Realgar — The Toxic Arsenic Sulphide Collected for Its Colour
An orange-red mineral so soft and chemically unstable that it can never serve in jewellery — and a textbook case in mineral specimen care
Realgar is an arsenic sulphide mineral with the formula As4S4, occurring in vivid orange-red to deep red crystals, frequently with adamantine to resinous lustre. The mineral is occasionally faceted by collectors despite extreme softness — Mohs hardness 1.5 to 2 — and the substantial toxicity that follows from its arsenic content. It has no commercial jewellery use; specimens are valued in the mineralogical and decorative-art markets and are handled with the precautions appropriate to a hazardous arsenic-bearing material. Realgar is also a textbook case in light-sensitive mineral degradation, since prolonged exposure to ultraviolet light converts it irreversibly to yellow pararealgar and ultimately to powdery arsenolite.
Mineralogy and chemistry
Realgar crystallises in the monoclinic system, typically as short prismatic crystals or as massive granular aggregates. The crystal habit is well-developed in fine specimens from the classic localities, with sharp prism faces and well-defined terminations on the better material. The colour ranges from intense orange-red through brick-red to nearly black-red in dense or weathered specimens. Specific gravity is high — around 3.56 — reflecting the heavy arsenic content. The streak is orange-yellow.
Chemically, realgar is the lower-sulphur arsenic sulphide; orpiment, As2S3, is the higher-sulphur arsenic sulphide and frequently occurs in association with realgar at the same localities. The two minerals together produce the strikingly contrasted orange-red and golden-yellow specimen pieces prized by mineral collectors, and historically they were ground and used as pigments — vermilion-orange and king's yellow respectively — in painting and textile work until the toxicity of arsenic was recognised and the materials phased out of artistic practice.
Light sensitivity and degradation
Realgar's most distinctive property from a curatorial standpoint is its light sensitivity. Prolonged exposure to ultraviolet light — including ordinary sunlight and many artificial lighting environments — causes the realgar lattice to rearrange to pararealgar, a yellow polymorph with the same chemistry but different crystal structure. The transformation is gradual but cumulative, and a realgar specimen displayed in normal lighting conditions for years to decades will progressively yellow and lose the original orange-red colour. Continued exposure beyond the pararealgar stage produces arsenolite, a white powdery oxide, in a process that ultimately destroys the specimen entirely.
Curators and collectors store realgar specimens in dark conditions or in display cases with UV-filtered glass, and museum loans of important realgar specimens are typically conditional on appropriate lighting. The phenomenon is one of the better-documented cases of light-induced mineral degradation and is referenced in mineralogical literature on specimen care.
Localities
Historic and current sources of realgar include Hunan and Yunnan provinces in China, where the Shimen and Jiepaiyu mines have produced the finest specimens of the past several decades; the Getchell mine in Nevada, source of large transparent crystals from the mid-twentieth century; Quiruvilca in Peru, with associated orpiment; the Romanian Carpathians; Hungary; and various smaller occurrences in the western United States and Mexico. The Chinese material from Shimen has dominated the high-end specimen market since the 1990s, with crystals reaching several centimetres on terminated prisms and showing the most intense colour saturation.
Toxicity and handling
Arsenic content is the principal handling consideration. Realgar contains roughly 70 per cent arsenic by mass, present in the relatively stable As4S4 molecular form. Casual handling of intact crystals presents a low immediate hazard; the principal risks arise from grinding, dust generation, prolonged skin contact, and ingestion. Curators handle realgar with gloves, store specimens in dust-tight containers, and avoid lapidary preparation that would generate inhalable arsenic-bearing dust. Ground realgar — historically used as the pigment vermilion in some traditions — is frankly toxic and should not be handled without appropriate protective equipment.
Some collectors and museums apply protective lacquer coatings to realgar specimens to slow both the light-induced degradation and the slow oxidation that occurs at humid conditions. The coatings extend specimen life but are visible under inspection and reduce the appeal of the most aesthetically pure specimens.
In the trade
Realgar trades in the mineral specimen market and has no role in the jewellery trade. Top-grade Chinese material from Shimen reaches prices in the thousands of dollars for fine cabinet specimens, and exceptional pieces with intact colour and clean termination on quartz or calcite matrix have reached significantly higher prices in specialised auction. The trade is small but active, with mineral dealers at Tucson, Munich, and other major shows handling realgar alongside the broader sulphide and rare-mineral inventory. For collectors, the appeal lies in the colour, the crystal habit, and the curatorial challenge of maintaining the specimen against light degradation; for working jewellers and gem dealers, the mineral is encountered only in passing reference and not as commercial inventory.