175. FLUOR SPAR. — OPAL.'
This mineral, although it occurs in such a variety of beautiful colours, is but rarely used as a precious stone. This is largely due to its comparative softness, but also, in all probability, to its abundance ; and when it is used it is too frequently with the prefix "false"—"false Topaz," " false Emerald," for instance. This is jarring to the lover of minerals, for why not accept its undoubted beauty for what it is worth, and under its own simple name?
The range of colour it shows is as varied as any mineral known, but the shades of most importance from the present point of view are those that are pure and well marked. Thus it is found of a rich blue, bright green, rose-red, sometimes almost ruby-red, a rich honey-yellow, lilac, amethystine-violet and many other colours.
Its lustre is usually vitreous, but may be almost adamantine; it varies from perfectly transparent to subtranslucent.
Refraction is normally single with a refractive index nr 1.43, but it often shows anomalous double refraction, which persists even at a dull red heat.
When heated gently it shows marked phosphorescence, the colour of the light emitted varying and having no relation to the colour in daylight. Some varieties give a distinct green light.
