Chalcedony

Group: Quartz Mineral class: Oxides Colour powder blue, blue, brownish white Pleochroism: absent Mohs’ hardness: 6.5-7 Density: 2.55-2.65 Cleavage: absent Fracture: concave, irregular Crystal system: trigonal, fibrous aggregates Transparency: translucent to opaque Lustre: wax lustre Fluorescence: bluish white

Its name comes from the antique harbour of Chalcedon, in the Bosphoros. Chalcedony is a relatively prodigous group of quartz aggregates, which include: - agate: white, yellow or reddish-brown - onyx: black and white stripes - cornelian: fleshy red - sard: brown and white stripes - chrysoprase: green, apple green - heliotrope: dark green with red spots - moss agate: colourless with green inclusions However, when we talk about the gemstone Chalcedony, we are referring to the so-called Authentic Chalcedony, from a blue-whitish colour. In antiquity it was highly prized as a talisman against melancholy.

It is made of microcrystals lined up parallel to the surface. If they are cut very finely, they can be semitransparent, although, normally, they are opaque and have a noticeably waxy glow. Due to its high porosity, it is very easy to colour although, and also, because of this, is very sensitive to acids and grease. If you are wearing a Chalcedony ring, you should be very careful while doing domestic work as the stone can change colour. Natural Chalcedony is evenly coloured but many synthetic Chalcedonies are blue and white striped.

In antiquity, the most important mines were in Bombay (India) and in Tuscany (Italy). Today, the most important deposits are in Uruguay and Brazil.

Chalcedony is ideal for cabochon cut, circular stone cut, sculptures and engraving works.

It can be confused with chrysoprase and some jadeites, but these are substantially softer than the chalcedony. There are also plastic imitations and some synthetic stones.