98 A POPULAR TREATISE ON GEMS.
times
becoming paler, or disappearing, as in chrysoprase and rose-quartz ; at
other times darker, as in brown spar, siderite, and rhodonite. In a few
minerals a complete change of color takes place, as in the
chlorophseite of the Western Isles, which, on exposure for a few hours,
passes from a transparent yellow-green to black. These mutations seem
generally connected with some chemical change. The tarnished colors
sometimes only appear on certain faces of a crystal belonging to a
peculiar form. Thus a crystal of copper pyrites (like fig. 35) has one
face P' free from tarnish ; the faces b and c, close to P', are dark blue; the remainder of c, first
violet, and then, close to P, gold-yellow. The color of the powder
formed when a mineral is scratched by a hard body is often different
from that of the solid mass. This is named the streak, and is
very characteristic of many minerals. It also often shows a peculiar
lustre where the mineral is soft, as in'talc and steatite.
Phosphorescence, Electricity, Magnetism.
Phosphorescence is
the property possessed by particular minerals of producing light in
certain circumstances without combustion or ignition. Thus some
minerals appear luminous when taken into the dark after being for a
time exposed to the sun's rays, or even to the ordinary daylight. Many
diamonds and calcined barytes exhibit this property in a remarkable
degree; less so, arragonite, calc-spar, and chalk; and in a still
inferior degree, rock-salt, fibrous gypsum, and fluor spar. Many
minerals, including the greater part of those thus rendered
-phosphorescent by the influence of the sun, also become so through
heat. Thus some topazes, diamonds, and varieties of fluor spar, become
luminous by the heat of the hand; other varieties of fluor spar and
the phosphorite require a temperature near that of boil-
