Euhedral refers to well formed crystals with sharp easily recognised faces. Normally crystals do not form smooth faces or sharp crystal outlines. Many crystals grow from cooling liquid magma, as magma cools and crystals grow they will 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 from familiar, euhedral, six-sided twinned crystals. In rocks, the presence of euhedral crystals may mean they formed first or perhaps crystalised in a cavity called a vug, without hindrance form other crystals.
