174 A Book of Precious Stones
and
of a dark-green colour. Quartz, as is mentioned elsewhere in
connection with its gem-stone varieties, crystallises in the hexagonal
system; hardness, 7; specific gravity, 2.0s to 2.8-—the
purest kinds 2.65. Pure quartz is silica; the varied colours and
characters of the many gem-stone varieties are due wholly or partly to
contents of iron, alumina, manganese, nickel, and other chromatic
constituents. The red spots in bloodstone are simply oxide of iron. The
specific name, heliotrope, is favoured by Dana, among other
mineralogists. " Heliotrope " is a word derived from two Greek words
meaning " sun-turning," and refers to the belief that the stone when
immersed in water would change the image of the sun to blood-red. The
water was also reputed to boil and upturn the experimental utensils
containing this submerged weird mineral.
This
opaque, but slightly lustrous, jaspery quartz, although a beautiful and
interesting mineral, is not extensively used now in jewelry, and a
requisition for it is usually an idiosyncrasy, or because it is a
natal stone for those who were born in the month of March. Hardy,
tough, yet carved with facility, it is well adapted to signet rings and
is usually seen bearing
