Conchoidal fracture
A conchoidal fracture is produced when some types of mineral, such as obsidian and flint, are broken. The fracture takes the form of a curved surface, reminiscent of the interior of a shell. This characteristic meant that these minerals were widely used in the stone age to make sharp tools. Ordinary glass also fractures conchoidally.
As conchoidal fractures can be produced only by mechanical impact, rather than frost cracking for example, they can be a useful method of differentiating prehistoric stone tools from natural stones.
A swelling appears at the point of impact called the bulb of percussion. Shock waves emanating outwards from this point leave their mark on the stone as ripples and the appearance in rather like a mussel shell, the Greek word for which conchoid derives. Other conchoidal fetaures include small fissures emanating from the bulb of percussion.
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