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.
The second physical property of light is seen in those stones which owe their beauty or value to Reflection: this again may be dependent on Lustre, or Colour.
Lustre.—This is an important characteristic due to reflection, and of which there are six varieties:—(α) adamantine (which some authorities, experts and merchants subdivide as detailed below); (β) pearly; (γ) silky; (δ) resinous; (ε) vitreous; (ζ) metallic. These may be described:—
(α) Adamantine, or the peculiar lustre of the diamond, so called from the lustre of adamantine spar, which is a form of corundum (as is emery) with a diamond-like lustre, the hard powder of which is used in polishing diamonds. It is almost pure anhydrous alumina (Al2O3) and is, roughly, four times as heavy as water. The lustre of this is the true "adamantine," or diamond, brilliancy, and the other and impure divisions of this particular lustre are: splendent, when objects are reflected perfectly, but of a lower scale of perfection than the true "adamantine" standard, which is absolutely flawless. When still lower, and the reflection, though maybe fairly good, is somewhat "fuzzy," or is confused or out of focus, it is then merely shining; when still less distinct, and no trace of actual reflection is possible (by which is meant that no object can be reproduced in any way to define it, as it could be defined in the reflection from still water or the surface of a mirror, even though imperfectly) the stone is then said to glint or glisten. When too low in the scale even to glisten, merely showing a feeble lustre now and again as the light is reflected from its surface in points which vary with the angle of light, the stone is then said to be glimmering. Below this, the definitions of lustre do not go, as such stones are said to be lustreless.
(β) Pearly, as its name implies, is the lustre of a pearl.
(γ) Silky, possessing the sheen of silk, hence its name.
(δ) Resinous, also explanatory in its name; amber and the like come in this variety.
(ε) Vitreous. This also explains itself, being of the lustre of glass, quartz, etc.; some experts subdividing this for greater defining accuracy into the "sub-vitreous" or lower type, for all but perfect specimens.