Actinolite forms a series with the minerals tremolite and ferro-actinolite within the amphibole mineral group. This group has been revised in recent years and readers need to consult up-to-date mineralogical literature to keep up with name changes. Actinolite gemstones, with the exception of nephrite (see Chapter 20), are usually small and dark green and are fashioned from bladed monoclinic crystals. The latest version of the composition is given by the 2004 edition of Fleischer’s Glossary of Mineral Species as Ca2 (Mg,Fe2 5 ) Si8O22OH2.
The hardness is near 5.5, the SG around 3.05 and the RIs in the range 1.620–1.642 with DR 0.022. Specimens may show yellow to dark green pleochroism and an absorption line at 503 nm. Actinolite gemstones found in Tanzania showed an RI for the alpha, beta and gamma rays respectively of 1.619–1.622, 1.632–1.634 and 1.642–1.644. Chatoyant material can be quite attractive; one cat’s-eye gave an SG of 3.00 and RI near 1.63. A brighter green than the commoner iron-coloured material is shown by a chromium-bearing variety to which the confusing name smaragdite was at one time given.