CHLORASTROLITE
This
mineral, the name of which means " green-star stone," is solely of
American occurrence, and thus far has been found at but a single
locality. It occurs at Isle Royale, an island in Lake Superior, in the
form of beach pebbles. These pebbles come from the adjoining
amygda-loidal trap rock, out of which they weather. They are opaque,
and of light, bluish-green color, with a mottled effect arising from a
stellated or radiated structure. This structure, when the stone is
polished, affords a chatoyancy which is very pleasing. It is especially
desirable in a good stone that the radiation should emanate from the
center, in which case a cat's-eye effect is obtained. The pebbles which
make good stones are mostly small; but some an inch in diameter are
known. The hardness of the mineral is 5.5, and its specific gravity
3.18. It is not a homogeneous mineral but a mixture, composed chiefly
of a hydrous aluminum silicate. The stones have not attained extensive
use as yet, except as they are sold in quantities to tourists in the
Lake Superior region. ^
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