In prehistoric times, cryptocrystalline stones such as chert, flint, obsidian, and chalcedony, as well as some finegrained rhyolites, felsites, quartzites and a few other tool stones, were often used as a source material for stone tools. These materials are easily broken and fracture in a Hertzian cone when struck with sufficient force. In this kind of fracture, a cone of force propagates through the material from the point of impact, eventually removing a full or partial cone; this result is familiar to anyone who has seen what happens to a plate-glass window when struck by a small object, such as an airgun projectile. The partial Hertzian cones produced during lithic reduction are called flakess, and exhibit features characteristic of this sort of breakage, including striking platforms, bulbs of force, and occasionally eraillures, which are small secondary flakes detached from the flake's bulb of force. Flakes are often quite sharp, with distal edges only a few millimeters thick, and can be used directly or modified into other types of tools such as spokeshaves and scrapers. Occasionally, obsidian flakes are used for delicate surgery instead of steel blades, due to their preternatural sharpness.