Cinnabar is mercury(II) sulphide, HgS, in its trigonal form, and it is the principal ore of mercury and the source of the red pigment vermillion used by painters and lacquerers from antiquity to the early twentieth century. The mineral is one of the most striking in the inorganic world: a saturated, slightly orange-tinged crimson-red, with a vitreous to adamantine lustre and a strong birefringence. It is occasionally cut as a faceted gemstone for collectors and is more commonly used in carved ornamental work, especially in the Chinese lacquer tradition, although the same toxicity that has driven mercury out of industrial use makes cinnabar a material to be handled with caution rather than worn loose against the skin.

Crystallography and physical properties

Cinnabar crystallises in the trigonal system in the space group P3₁21 or P3₂21, and forms rhombohedral, prismatic, or thick tabular crystals, often twinned. The colour is a strong vermillion-red, the streak scarlet, the lustre adamantine when crystalline and earthy when massive. Hardness is unusually low at 2 to 2½ on the Mohs scale, and the specific gravity is correspondingly high at 8.05 to 8.20, reflecting the heavy mercury content - one of the heaviest minerals among gem species. Refractive indices are extraordinarily high at 2.905 and 3.256, with a birefringence of 0.351, the highest of any common gem material; the optic character is uniaxial positive. The high refractive indices give cinnabar an almost diamond-like fire when faceted, but the low hardness limits practical use as a wear stone.

Geological occurrence

Cinnabar is a low-temperature hydrothermal mineral, deposited in fissures and replacement bodies in volcanic and sedimentary host rocks at temperatures generally below 200°C. The classical localities are Almadén in central Spain, the world's largest mercury deposit and worked continuously from Roman times until 2003; Idrija in Slovenia, second only to Almadén in historical importance and worked from 1490 to 1995; Monte Amiata in Tuscany; Huancavelica in Peru, exploited heavily under the Spanish colonial silver-amalgamation industry; Wanshan and Tongren in Guizhou, China, which produced the cinnabar carved into Chinese lacquerware; the New Almaden and New Idria mines of California; and a constellation of smaller occurrences from the Philippines to Algeria.

The pigment vermillion

Cinnabar's most consequential historical use is as the source of vermillion, prepared either by grinding natural cinnabar or by synthesising mercuric sulphide from elemental mercury and sulphur, a process recorded in the recipe-books of medieval Europe and earlier in Tang dynasty China. The pigment was the most important brilliant red of the European medieval and Renaissance painter's palette, used in illuminated manuscripts, panel paintings, and frescoes; it appears in works from the Lindisfarne Gospels through the Limbourg brothers' Très Riches Heures to Botticelli, Raphael, and Titian. In East Asia vermillion was used in lacquer, in temple architecture, and in the imperial seals of the Chinese emperors, where the colour came to signal imperial authority.

Chinese lacquer carving

The most distinctive use of cinnabar in ornament is the Chinese tradition of carved cinnabar lacquer, in which dozens or hundreds of layers of cinnabar-pigmented lacquer are applied to a wooden core and then carved in deep relief. The technique reaches its peak in the Ming and early Qing dynasties, with imperial workshops at Beijing producing carved cinnabar boxes, trays, screens, and ceremonial objects of extraordinary refinement. The technique has continued into the twentieth and twenty-first centuries, and modern "cinnabar" lacquer is now often made with synthetic vermillion or with substitute red pigments rather than mercury sulphide, in part because of toxicity regulations. Disclosure conventions in the antique trade vary, and the buyer should not assume that a piece described as cinnabar is in fact pigmented with mercury sulphide unless the description is specific.

As a faceted gemstone

Faceted cinnabar is a connoisseur's stone. Transparent crystals of useable size are rare and come principally from China and from a small number of finds in Spain and Slovenia. When cut, the stone shows extraordinary fire and a saturated red colour, but the low hardness, the cleavage, and the toxicity restrict it to the curio cabinet and the locked specimen tray. Sizes above one or two carats are unusual; the largest faceted cinnabars in the published gem literature are in the high single digits of carats. The stone is sometimes confused with proustite (silver sulpharsenide) and with the rarer red mercury minerals, and a refractive-index measurement (where the indices exceed the range of standard gem refractometers) and a specific-gravity test resolve the question.

Toxicity and handling

Cinnabar is mercury sulphide. The compound itself is relatively insoluble and not strongly bioavailable through brief skin contact, but the dust is dangerous to inhale, the mineral can release elemental mercury when heated, and any prolonged contact with acids or warm conditions is to be avoided. Faceters and carvers working with cinnabar use wet methods to suppress dust and dispose of waste through hazardous-material channels. For wear, cinnabar is set under closed backs or behind glass; loose stones in everyday jewellery are not advised. The same caution applies to old vermillion-pigmented lacquer pieces showing surface wear or flaking.

Place in the trade

Cinnabar occupies a small but recognised place in the gem and ornament trade: it is collected as a mineral specimen, valued in the form of historical Chinese lacquer, faceted occasionally for connoisseurs, and bought for its colour, weight, and historical resonance. The closure of Almadén in 2003 marked the effective end of major mercury mining and converted the remaining cinnabar trade from an industrial by-product to a strictly ornamental and collector category, where it is likely to remain.