CHAPTER V.
PHYSICAL PROPERTIES.
C—Light.
Probably the most important of the many important physical properties possessed by precious stones are those of light and its effects, for to these all known gems owe their beauty, if not actual fascination.
When light strikes a cut or polished stone, one or more of the following effects are observed:—it may be transmitted through the stone, diaphaneity, as it is called; it may produce single or double refraction, or polarisation; if reflected, it may produce lustre or colour; or it may produce phosphorescence; so that light may be (1) transmitted; (2) reflected; or produce (3) phosphorescence.
(1) Transmission.—In transmitted light we have, as stated above, single or double refraction, polarisation, and diaphaneity.
To the quality of refraction is due one of the chief charms of certain precious stones. It is not necessary to explain here what refraction is, for everyone will be familiar with the refractive property of a light-beam when passing through a medium denser than atmospheric air. It will be quite sufficient to say that all the rays are not equal in refractive power in all substances, so that the middle of the spectrum is generally selected as the mean for indexing purposes.
It will be seen that the stones in the 1st, or cubic system, show single refraction, whereas those of all other systems show double refraction; thus, light, in passing through their substance, is deviated, part of it going one way, the other portion going in another direction—that is, at a slightly different angle—so that this property alone will isolate readily all gems belonging to the 1st system.
A well-known simple experiment in physics shows this clearly. A mark on a card or paper is viewed through a piece of double-refracting spar (Iceland spar or clear calcite), when the mark is doubled and two appear. On rotating this rhomb of spar, one of these marks is seen to revolve round the other, which remains stationary, the moving mark passing further from the centre in places. When the spar is cut and used in a certain direction, we see but one mark, and such a position is called its optical axis.