About the Optical Properties of Pleichroism in Gem Stones

The dichroiscope is a handy little optical instrument, that will readily serve to distinguish the diamond, spinel, or garnet (all singly refracting minerals) from the ruby, beryl, or any of the doubly refracting stones. This instrument consists of a cleavage rhombohedron of Iceland spar, fastened in a brass tube about 2 1/2 inches long, and 3/4 of an inch in diameter. A sliding cap at one end has a perforation 1/8 of an inch square, and at the other end is a lens which will show a distinct image of the square opening when the cap is pulled out about 1/4 of an inch.

The pleiochroism of many stones can be determined at a glance with the dichroiscope.

When a stone is examined by means of the dichroiscope, it will show two images of the same hue, or of different hues, these square images (fig. 1, A) forming a right angle and being more distinct when viewed from one part of the stone than from another.

When the images are identical in color, the specimen may be a diamond, garnet, spinel, or glass. Should a red or ruby spinel approach the ruby in color, a quick and satisfactory test can be made with the dichroiscope, as the spinel will show two images of one color, while the ruby will show one image of aurora red and one of carmine red.

The dichroiscope is inexpensive, costing but a few dollars, and is very useful for rapidly deciding the species of many stones. The following is a partial list of doubly refracting stones and their twin colors.

NAME OF STONE. TWIN COLORS.

Sapphire (blue) Greenish straw Blue

Ruby (red) Aurora red Carmine red

Tourmaline (red) Salmon Rose pink

Tourmaline (brownish red) Umber brown Columbine pink

Tourmaline (brown) Orange brown Greenish yellow

Tourmaline (green) Pistachio green Bluish green

Tourmaline (blue) Greenish gray Indigo blue

Emerald (green) Yellowish green Bluish green

Topaz (sherry) Straw yellow Rose pink

Peridot (pistachio) Brown yellow Sea green

Aquamarine (sea green) Straw white Gray blue

Beryl (pale blue) Sea green Azure blue

Chrysoberyl (yellow) Golden brown Greenish yellow

Iolite (lavender) Pale buff Indigo blue