Chert rock is common in the "Edwards" geologic formation here in central Texas. A more common layman term is "flint". Chert is a rock (not a mineral) composed of microscopic quartz crystals (microcrystalline). Chert has a smooth surface and fractures with a conchoidal shape. Chert is found as nodules (subrounded mass) or layers of limited horizontal extent within limestone rock. Chert occurs in many colors such as white, yellow, red, brown, green, blue, gray, and black. The yellowish, red, and brown chert is commonly called "jasper", while the dark gray to black chert is commonly called "flint".
The conchoidal fracturing is what made "flint" a popular rock type for carving an otherwise very hard material and making arrow heads, spear tips, or knives. The image at right includes an arrowhead made of chert found in Georgetown. The other two items are very dark gray to black chert rock fragments (flint).
