Loose Tremolite For Sale Online At Wholesale Prices. Tremolite is a fibrous mineral belonging to the Amphibole group. It is a metamorphic mineral with some of the nicest material coming from Tanzania. On Gem Rock Auctions we are lucky enough to have a small range of specimens for sale direct from Tanzania. The green colour is caused by the impurity Iron. The higher the content of Iron, the greener the stone will become. It measures 5 – 6 on the MOHS scale of hardness making it soft so special care should be taken.