| Silica | 42.13 |
| Alumina | 36.43 |
| Boracic acid | 5.74 |
| Oxide of manganese | 6.32 |
| Lime | 1.20 |
| Potash | 2.41 |
| Lithia | 2.04 |
The green tourmaline is composed of
| Silica | 40. |
| Alumina | 39.16 |
| Lithia and potash | 3.59 |
| Protoxide of iron | 5.96 |
| Protoxide of manganese | 2.14 |
| Boracic acid | 4.59 |
| Volatile matter | 1.58 |
The tourmaline possesses double refraction to a high degree, and its power of polarizing light is so great that, cut into slices, it is used in the polariscope for analyzing other minerals.
If two slices of tourmaline, cut parallel to their axis, be laid one on the other in one direction, both are transparent; if laid in another direction they become opaque, and if a doubly refracting crystal be placed between the two plates of tourmaline, the part covered by the crystal is transparent while the other is opaque.
Tourmaline melts with borax into a transparent glass; the rubellite turns white, and the indicolite and green tourmalines turn black, under the blow-pipe.
Tourmalines can be distinguished from other gems by their specific gravity, but principally by their property of assuming polaric electricity after being heated, one end becoming positive and the other negative.
The history of the discovery of the tourmaline and its electric property is a curious one.
On a warm summer day, early in the eighteenth century, some children were playing in a courtyard in Amsterdam. Amongst their playthings were some precious stones which the Dutch navigators had brought from Ceylon. Some of the stones seemed to be possessed of the strange power of attracting and repelling small bits of straw, ashes, and other light substances. The little ones called their parents to witness this strange phenomenon, and the stolid Dutch lapidaries, themselves puzzled at the sight, called the stones aschentreckers or ash-drawers.
A number of years afterwards, careful experiments disclosed the wonderful electric powers of the aschentreckers or tourmalines. Purple, green, and blue tourmalines are found in Brazil. In Ceylon the stones are found in gravel beds. Rubellites or siberites are found in Siberia.