Fragments of prismatic crystals of tourmaline embedded in a pegmatite host rock. Tourmaline is a boron-silicate mineral of variable colour, some being much sought after as semi-precious gemstones. It usually occurs in igneous rocks. Common varieties are elbaite, dravite and schorl. The prisms in this sample, nearly opaque and very black as they are, are more of the schorl variety. Specimen is from the Antandrokomby pegmatites in the Valley of Madagascar, well known for containing precious and semi-precious minerals such as tourmaline and beryl. Actual width of specimen is 65 centimetres. See pocket knife for scale.