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 Umbalite Garnet rough from the Umba Valley weighs 14.19 cts and measures 15x13x7 mm.