234. SPINEL AND CHRYSOBERYL.
Spinel included
some of the gems called Lychnis by Pliny, and his Carbunculi
Amethystizontes were Spinels also. Though it has been known as a gem
stone since remote times, it was till the eighteenth century confused
with other species.
As
met with in jewellery, the characteristic colour is flame-red; this
form is sometimes distinguished as Spinel Ruby or Ruby Spinel; lighter
rose-red shades are called Balas Ruby; specimens of a violet tint are
known as Almandine Spinels or sometimes Alabandin Ruby (from Alabandin
in Caria); blue stones are known as Spinel Sapphire, and the orange-red
ones as Rubicelle; a colourless variety is sometimes seen. Other
colours occur in the mineralogical varieties, Pleonaste, Chlorospinel
and Picotite.
The
lustre of the gem varieties is vitreous, and often splendent, and they
are transparent or subtransparent; the other varieties may occur
almost, if not quite, opaque.
It
is singly refracting, hence shows no dichroism, and under the
polariscope the field remains dark when the specimen is rotated under
crossed Nicol's prisms. It phosphoresces with a red light. Its index of
refraction for yellow light is 1.72, and the dispersion is small.
On rubbing it shows a surface charge of positive
