Obsidian is an extrusive volcanic glass formed from the rapid cooling of silica-rich magma, leaving little time for crystallization to occur. This igneous rock uniquely lacks any crystalline structure and has a distinct glassy appearance and a conchoidal fracture, as seen in sample A. Though it is typically dark and looks mafic rocks in colour, obsidians are of felsic composition. Sample B is obsidian layered with rhyolite, a light-coloured aphanitic igneous rock of silica-rich, felsic magma. The layering in sample B infers a zone of high volcanic activity where volcanic deposits are recycled leading to different crystallization rates. Since obsidian is glass and less resistant to weathering, there is little obsidian found older than Cretaceous in age. Obsidian is widely used as gemstones or for ornamental purposes but had once been a material that was mined, manufactured, and traded to be cutting tools and weapons. Its conchoidal fracture allows for extremely sharp edges when broken and sculpted so it was useful as knives, arrowheads and even continues to be used in some modern practices as a surgical tool.