Description

Tourmaline is a crystalline boron silicate mineral compounded with elements such as aluminum, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone, and the gemstone can be found in various colors. Tourmaline crystals are highly birefringent, a crystallographic feature resulting in the split of a ray of light crossing the crystal outside its optical axis into an ordinary and an extraordinary ray, being polarized at right angles to each other. However, the E-ray is rapidly absorbed, so if the crystal is a few millimeters thick, the transmitted light is linearly polarized. Tourmaline tongs is a primitive polariscope device having a transparent tourmaline crystal thin section in each half, mounted in cork in independently rotatable oxidized brass disks at the ends of a pair of spring tongs mounted with the transmission axes at right angles to each other. Specimens, such as the mineral crystal specimens mounted in square paper-covered cork plates (provably by Steeg & Reuter) seen here, are slipped between the two tongs and observed by holding the system up to the eye. These specimens contain each a crystal slice set in a specific vibration axis of its optical indicatrix to yield an interference figure (seen in the photos taken with a cellphone camera through the instrument seen here). The device was invented by Jean-Baptiste Biot (1774-1862), as an inexpensive, effective, and easy-to-use polariscope. In 1816, the Scottish physicist, David Brewster (1781-1868), had devised a simple polariscope in which the object to be examined was held between two plates of a singly refracting crystal (he used agate) placed transversally to one another. This early experimental work in physical optics, mostly concerned with studying the polarization of light and the birefringence of crystals, was the first step in creating the field of optical mineralogy.