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
(2) reflected; or produce
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.
Polarisation is when certain crystals possessing double refraction have the power of changing light, giving it the appearance of poles which have different properties, and the polariscope is an instrument in which are placed pieces of double-refracting (Iceland) spar, so that all light passing through will be polarised.
Since only crystals possessing the property of double refraction show polarisation, it follows that those of the 1st, or cubic system—in which the diamond stands a prominent example—fail to become polarised, so that when such a stone is placed in the polariscope and rotated, it fails at every point to transmit light, which a double-refracting gem allows to pass except when its optical axis is placed in the axis of the polariscope, but this will be dealt with more fully when the methods of testing the stones come to be considered.
Diaphaneity, or the power of transmitting light:
—some rather fine trade distinctions are drawn between the stones in this class, technical distinctions made specially for purposes of classification, thus:
—a "non-diaphanous" stone is one which is quite opaque, no light of any kind passing through its substance; a "diaphanous" stone is one which is altogether transparent; "semi-diaphanous" means one not altogether transparent, and sometimes called "sub-transparent."
A "translucent" stone is one in which, though light passes through its substance, sight is not possible through it; whilst in a "sub-translucent" stone, light passes through it, but only in a small degree.
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