Tabasheer

Tabasheer, a substance sometimes found in the cavities or tubular parts of the stems of bamboos and other large grasses. It consists chiefly of silica with a little lime and vegetable matter, or sometimes of silica and potash, in the proportions of about 70 parts of silica and 30 of potash. It appears to be formed by extravasation of the juices of the plant, in consequence of some diseased condition of the nodes or joints. It is in high repute among the Hindus as a tonic, and is prepared by imperfect calcination and trituration. The powder is often chewed with betel in order to renovate the constitution. There are several varieties of tabasheer, one of which, of very rare occurrence, is extremely beautiful, of a delicate azure colour by reflected light, and of a faint yellowish line by transmitted light, easily crushed between the fingers, and of 'an aerial and unsubstantial texture, which we look for in vain in any other solid.' Other varieties are yellowish, white, and much like some varieties of opal. Tabasheer is very porous, and absorbs water and oil very rapidly; effervescence taking place when it is plunged in water. By absorption of oil the opaque varieties become transparent. When the greater part of the oil is expelled by heat the structure of the tabasheer becomes apparent; it is beautifully veined, the veins being sometimes parallel and sometimes curved. The optical properties of tabasheer are remarkable. Of all known substances it has the lowest refractive power.