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Flint and obsidian were very important Stone Age materials. Commonly found with chalk and limestone, flint is a form of the mineral quartz. Obsidian is a naturally occurring volcanic glass. Both were widely used in weapons and tools. As we will learn in this lesson, flint and obsidian are classic examples of ceramics. Both are hard and can be worked to produce a sharp edge, but both materials are prone to breakage. Slowly heating flint to 150 to 260 °C (300 to 500 °F), holding the temperature there for 24 hours (annealing), and then slowly cooling it back to room temperature, can relieve internal stresses which can improve the ability to produce flint tools or weapons with a sharper cutting edge. As discussed later, since flint is typically found with chalk and limestone, it is possible that the annealing of flint led to the discovery of lime mortar.