Tourmaline. Source: http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/photogallery/ archive copy at the Wayback Machine ; English Wikipedia
Schist photomicrograph.jpg English: Figure caption: Antietam Schist. Porphyroblastic biotite, a, in a dolomitic matrix. The tourmaline, t, is epigenetic.
