And Jeff Kurtzeman took Best of Species in the thumbnail category with a green penetration twin
fluorite from County Durham, England.
Crystals are typically cubic, in some cases showing penetration twins
formed by rotation about the three-fold axis--a habit common in Weardale-area fluorite.
The cinnabar crystals of the Wanshan-Yanwuping area are typically dark red or scarlet red penetration twins
associated with dolomite and quartz crystals (Liu, 1995).
The illustration on the right is of a symmetric contact twin, rather than the penetration twins
which are the focus of this paper.
Penetration twins, however, can be mistaken for single crystals.
The situation is different with penetration twins. Twinned crystals develop in such a way that only (001) or (101) faces of the twinned individuals are present.