The best opals are the result of a flaw in their formation. Being hydrated silicas, they were at first a sort of semi-liquid, jellified substance; as this hardened, cracks and fissures were created by unevenness in the material and in the speed of the hardening. These tiny spaces trapped air or moisture, and it is this that produces the phenomenon of refraction and reflection of light and gives the colorings and variations known as opalescence. The play of light is at its best when the stone is cut cabochon, except for the fire opal, which is faceted.
There are three chief varieties of opal. The common or white opal has a cloudy-white background, with pastel patches that often give it a veritable sunrise glow. The black opal has actually a very dark green background, in which there are deep pools of blue and green with patches of flame. Rare, and most magnificent, is the fire opal, which seems almost transparent, its body of smooth reddish orange shooting forth into flame.
The opal is a delicate stone. It may be damaged by heat. It absorbs grease, and may thus become dull. The outstanding and valued feature of the stone is its opalescence. This creates a constantly changing, almost kaleidoscopic play of lights. It is this variability that gives point to the reference in Shakespeare’s Twelfth Night: “Now the melancholy god protect thee, and the tailor make thy garment of changeable taffeta, for thy mind is a very opal.”
Peridot
The peridot, a yellowish-green variety of chrysolite, was popular in early England. It fell from favor but was reintroduced from France in the seventeenth century. It is a beautiful stone, often as large as 30 carats, and again growing in favor.
Quartz
Quartz is one of the silicas, chemically Si O₂, as is the opal. It is abundant as a colorless, transparent substance; it also appears as a brilliant crystal. The name quartz is from the German zwerg, meaning dwarf. Similarly cobalt and nickel are from German words for sprites, the gnomes being little creatures that work the mine of the gods.
In its crystalline form quartz includes amethyst, cairngorm, citrine, quartz cat’s-eye, rock crystal, and rose quartz. Another main group in the quartz family is chalcedony, which includes agate, bloodstone, carnelian, jasper, moss agate, onyx, sard, and sardonyx. These stones are used for beads for carving cameos and intaglios.