Chalcedony

Chalcedony, named from Chalcedon in Asia Minor, is a crypto-crystalline mixture of quartz and opal. It occurs in stalactitic or mammillated forms, in flint-nodules, and in agates, with a radiating acicular crystallisation at right angles to the surface of the bosses. It is rather waxy in lustre, and varies much in colour, carnelian (q.v.) or sard being the red or brownish-red variety; prase, leek-green; plasma, the same colour, but more translucent; chrysoprasc, an apple-green; onyx, a regularly-banded form, white and bluish-grey; and sardonyx, similar, but red and white.