Calcites from the silver mining district around Freiberg in Germany are historic classics and, in fact, had a huge part in early mineralogy as scientists studied the various habits, including twinned crystals, to try to understand what a crystal actually was and how it was formed of structures within. Old German-language, French, and English books often show plates of different calcite habits, including such rare twins as this fishtail-twinned crystal, which probably also dates to the 1800s heyday of mining here. It is a moderately-lustrous, translucent, colorless, twin crystal with a modified termination, and is complete all around. For a German calcite thumbnail, it is outstanding.