[@Jangel13]

Knapping is the process used to sharpen flint and obsidian into blades by using careful strikes from another stone.  Stones that break in that manner are the class of stones that I'm referring to.  Most are igneous in origin, but it isn't usually too uncommon for them to show up where water has cut a path or where other geological upsets have occurred.  Of course, there are a wide range of stone types that can be sharpened in this way and their quality ranges widely as well.