Fire agate

Fire agate is a brown or reddish-brown, grape-like (botryoidal) variety of chalcedony. Extremely thin layers of iron oxide inside it reflect and refract light, producing red, golden orange and green, and sometimes blue and violet, flashes that seem to come from deep within the stone. Despite the word “agate” in its name it is usually unbanded, so strictly speaking it counts as a kind of chalcedony. Its only known deposits are in central and northern Mexico and in the US states of Arizona, California and New Mexico.

Jewellery: It is hard and takes a good polish, so it can be used in almost any kind of jewellery, such as rings, pendants and earrings. It is not faceted; it is made into freeform cabochons that follow its natural shape. Collecting: Rough and polished pieces are sought by collectors and museums. Rockhounding and local craft: Some areas in Arizona and California are open to rock collectors. In the Mexican state of Aguascalientes, cutting and polishing the stone has become a local craft.

It has a hardness of 6.5–7 on the Mohs scale. It scratches glass and steel; a steel knife cannot scratch it. Its specific gravity is 2.6–2.64 g/cm³.

Its best-known localities: Black Hills Rockhound Area, Graham County, Arizona, USA; Deer Creek, Galiuro Mountains, Graham County, Arizona, USA; Aguascalientes State (El Terrero workings and Calvillo area), Mexico; Opal Hill Mine, Mule Mountains (near Palo Verde), Riverside County, California, USA; Chihuahua State, Mexico.

On a brown background, red, orange, golden yellow and green flashes shift as the stone is turned; blue and violet are rarer. The surface is made of small bumps like a bunch of grapes; the colours follow the shape of these bumps and seem to come from inside the stone rather than from its surface. It belongs to the quartz family and is hard (6.5–7): it scratches glass. It is harder than opal.

On the “street to diamond” value ladder it sits on step 6 of nine: Fine ornamental and collector stone. What matters most is the colour, brightness and coverage of the “fire”: stones showing many colours together are sought after, and blue and violet are the most prized. Cutting skill is crucial: because the coloured layer is so thin, a stone that is ground too far loses its fire, and a good stone is worked by hand into a freeform cabochon that follows the pattern. Size, freedom from cracks and a famous source also matter to collectors. With so few deposits, it is rarer than ordinary agate.

Today crystal-therapy websites claim that fire agate forms a protective shield around its wearer and reflects harm back to where it came from. The same sources claim that the stone gives courage and self-confidence, boosts vitality and even rejuvenates the body. Fire agate is made of chemically inert silica with a little iron oxide, and its colours come from light interfering in thin layers. There is no scientific evidence that it protects, gives courage or vitality, or rejuvenates the body. Because fire agate was only recognised in the mid-20th century, it also has no tradition of belief reaching back to ancient times. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.