Refractivity

But it is not the hardness of the diamond so much as its optical qualities that make it so highly prized. It is one of the most refracting substances in nature, and it also has the highest reflecting properties. In the cutting of diamonds advantage is taken of these qualities. When cut as a brilliant the facets on the lower side are inclined so that light falls on them at an angle of 24° 13´, at which angle all the incident light is totally reflected. A well-cut brilliant should appear opaque by transmitted light except at a small spot in the middle where the table and culet are opposite. All the light falling on the front of the stone is reflected from the facets, and the light passing into the diamond is reflected from the interior surfaces and refracted into colours when it passes out into the air, giving rise to the lightnings, the effulgence, and coruscations for which the diamond is supreme above all other gems.

The following table gives the refractive indices of diamonds and other bodies:

Refractive Indices for the D Line
Chromate of lead2·50–2·97
Diamond2·47–2·75
Phosphorus2·22
Sulphur2·12
Ruby1·78
Thallium glass1·75
Iceland spar1·65
Topaz1·61
Beryl1·60
Emerald1·59
Flint glass1·58
Quartz1·55
Canada balsam1·53
Crown glass1·53
Fluor-spar1·44
Ice1·31

In vain I have searched for a liquid of the same refraction as diamond. Such a liquid would be invaluable to the merchant, as on immersing a stone the clear body would absolutely disappear, leaving in all their ugliness the flaws and black specks so frequently seen even in the best stones.