Better known as a constituent of the darker colored nephrite jades, transparent green and brown crystals of actinolite have been found in sufficient size to produce faceted stones. Varying from pale green to green to brown, the crystals form bladed crystals in the monoclinic system. One of the amphibole group of minerals, actinolite has the formula Ca2(Mg,Fe)5(OH)2(Si4O11); the crystals have a hardness of 5.5 on Moh’s scale, and there are two directions of cleavage.
With an SG of 3.05, a refractive index of 1.620 – 1.642, and a birefringence of .022 the gemological properties of actinolite are almost identical to tourmaline and cut stones are very difficult to identify with ordinary gemological tests. Since actinolite is quite rare, most of the stones could easily be misidentified as tourmaline. However, well equipped labs using IR spectral (FTIR), and (EDAX) chemical analysis are able to conclusively identify the stones without much difficulty.
The pleochroism is normally yellow to dark green and there is a faint absorption line at 503nm. Some stones are also found to produce cats eyes and long slender needles of actinolite are sometimes found as inclusions in quartz. A chromium-bearing actinolite called smaragdite of emerald green color is found in Tanzania.