Flint deserves a closer look. A gray-white beach stone with a chalky skin and a waxy, dark core can be flint, though waves can blur the clues and make chert, quartzite, or slag look persuasive. Look for curved breaks. Flint is a sedimentary form of microcrystalline quartz, commonly called chert in broader geological use; its silica structure can produce conchoidal fracture rather than grainy crumb...More