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In some exceptional cases, e.g. axinite, prehnite, &c., there is no apparent relation between the distribution of the pyro-electric charges and the symmetry of the crystals.

In its occurrence in basic rather than in acid eruptive rocks, axinite differs from the boro-silicate tourmaline, which is usually found in granite.

The hardness of 6�-7, combined with the colour and transparency, renders axinite applicable for use as a gemstone, the Dauphin� crystals being occasionally cut for this purpose.

I think we may fairly assert that such minerals as tourmaline, jargoon, peridote, spinel and chrysoberyl, though their names may be familiar, are not stones which would be recognized by any but those who are in some sense experts; while other minerals, such as sphene, andalusite, axinite, idocrase and diopside, are possibly almost unknown to most people, even by reputation.

The phenomenon was first discovered in the tourmaline, and it is observed, speaking broadly, only in those minerals which are hemimorphic, that is, where the crystals have different planes or faces at their two ends, examples of which are seen in such crystals as those of axinite, boracite, smithsonite, topaz, etc., all of which are hemimorphic.