Appearance
Simple crystals of alstonite are not known. The crystals are invariably complex twins formed by repeated twinning, and have the form of doubly terminated pseudo-hexagonal pyramids, like those of witherite but more acute; the faces are horizontally striated perpendicular to the pseudohexagonal c crystal axis and they are divided vertically by a medial, slightly reentrant twinning line parallel to the pseudohexagonal c axis.
Crystals are colourless to snow white, yellow-gray, pale gray, pale cream, pink or pale rose-red, but the colour may fade on exposure to light. They are transparent to translucent with a white streak and vitreous lustre. The examination in polarized light of a transverse section shows that each compound crystal is built up of six differently oriented individuals arranged in twelve segments.
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