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PRECIOUS STONES.

The specific gravity of tourmalines varies between 3 and 3.25. The following determinations were made with particular care:

Tourmaline, almandine coloured 3·009

Tourmaline, rich rose pink 3·044

Tourmaline, orange-brown, from Ceylon 3·082

Tourmaline, lemon yellow 3·106

Tourmaline, green, from Brazil 3·109

Tourmaline, black, Bovey Tracey, Devon 3·120

Tourmaline, green, from Brazil 3·154

The tourmaline occurs crystallised in the form of prisms belonging to the rhombohedral system; some of the faces are striated or even channelled. The hardness of perfectly flawless transparent tourmalines is from 7·3 up to 7·5.

The optical properties of tourmaline are most striking. When a crystal is viewed along the direction of its principal axis, it is less transparent and of a different colour than when viewed across that axis. The coloured varieties, or most of them, absorb and quench to different degrees the ordinary ray, which is polarised in a plane parallel to the axis, while they allow the extraordinary ray, polarized in a plane perpendicular to this line, to pass. Examples of the marked dichroism, which is so conspicuous a feature in the majority of coloured tourmalines, may be seen in this list of twin colours of the two polarized rays passing along and across the crystal respectively:

ORDINARY RAY.

Yellow brown.

Deep violet brown.

Purple.

EXTRAORDINARY RAY.

Asparagus green.

Greenish blue.

Blue.

The following are some additional instances of the twin colours seen in tourmalines, owing to the optical peculiarity just named. These examples were observed with the aid of the dichroiscope, which serves for the study of such a phenomenon admirably, causing, as it does, a complete separation of the oppositely-polarized and