UMF Authors Mark Brockmann and Barry Keegan:
This book describes many of those high-quality, siliceous, lithic (rock and mineral) materials used by the Indigenous people of Northeastern North America (NENA). The rocks and minerals include chert, hornfels, rhyolite, quartz, welded tuff, qaurtzite, jasper, chalcedony, agate and others...The Paleoindian lithic types most favored were very fine grained to aphanitic, high silica content, rocks and minerals. It was important that their lithic materials were extremely strong, durable; and would produce a distinctive conchoidal fracture, when knapped.
From Brockmann and Keegan