Glossary 347
in carbonic acid gas, when burned between the poles of a powerful electric battery.
Diaphaneity. The property of transmitting light.
DICHROISM.
A property of all doubly refractive stones, of which the two images
revealed by an instrument called dichroiscope appear in different
colours.
DiCHROSCOPE. An instrument designed to exhibit the two complementary colours of polarised light—the di-chroism of crystals.
Dispersion. The
power which decomposes a ray of common white light in its passage
through a transparent medium and splits it into the various colours of
which it is composed.
Dodecahedron. A
geometrical form in the isometric or cubic system applied to
crystallography; a solid figure of twelve equal sides, each a regular
pentagon —of five equal sides and angles.
DOLOMITIC. Pertaining to dolomite, a brittle, translucent mineral of various colours and a vitreous lustre.
Eruptive. Minerals of volcanic origin in geological formations.
Facet. One of the small planes which form the sides of a natural crystal, or of a cut diamond or other gem.
Fales. Stones of two, or more, differently tinted strata.
False Colour. Effect of " False Stones."
Fancy. A term that has been applied to semi-precious stones prized for other qualities than intrinsic value.
Fault. Anything
within, or on the surface of, a precious stone which detracts from its
beauty or value; obvious examples are inclusions of foreign bodies and
patches of a different colour or shade from the body of the gem.
Feathers. White subtransparent lines in the body of a stone.
Feminine. Eubies of a pale tint.
Ferrous. Any mineral substance having a considerable portion of iron in its composition.
Fire. Term applied to the lustre and brilliancy of gems, pre-eminently the diamond, and secondarily the opal.
