Garnets are a group of silicate minerals, Garnet forms when a sedimentary rock with high aluminium content, such as shale, is metamorphosed. Garnet has a hardness of 6.5 to 7.5 on the Mohs scale.This piece of Mahenge Garnet rough from Tanzania, Africa weighs 6.33 cts, measures 11X9X7mm and has some small viels.