Banded Iron Formation

The black parts of this polished slab are thin layers of dark, semimetallic hematite, and the red layers are jasper, an iron-rich chert. The gray and bright golden areas are part of the same layers, made of tiger-eye quartz. It forms when quartz replaces the fibrous mineral crocidolite (also known as blue asbestos). The dark mineral flashes in golden highlights as you turn it in the light, an effect called chatoyancy. (learn more about BIF)