Euhedral
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Euhedral refers to well-formed crystals with sharp, easily-recognized faces. Normally, crystals do not form smooth faces or sharp crystal outlines. Many crystals grow from cooling liquid magma. As magma cools, the crystals grow, and they eventually touch each other, preventing crystal faces form forming properly or at all.
However, when snowflakes crystallize, they do not touch each other. Thus, snowflakes form euhedral, six-sided twinned crystals. In rocks, the presence of euhedral crystals may signify that they first formed or perhaps crystalised in a cavity called a vug, without hindrance from other crystals.