Crystallized dodecaëdral Quartz

The regular dodecaëdral crystal of quartz is somewhat rare. I at present know of no certain habitat for it in Great Britain, excepting at Craig Lackart, about 3 miles Edinburgh, from whence I have an irregular group given me by Dr. P. Murray, who gathered it himself. It is evidently taken from a rock externally in a state of decomposition, as its matrix is porous and mixed with red oxide of iron. It is sometimes found at Bristol, and also on Lancashire iron ore or hæmatites, Bab. 80. I have such specimens also on an iron ore from Devonshire. The specimen here figured, I believe, is from Cader Idris in North Wales, and seems to have been thrown off from the main rock by an ochraceous decomposition: on that side towards the rock it is extremely porous, not unaptly resembling French burr, which is used for mill stones. Quartz or silex is not only common in our primitive mountains, but also in our gravel roads. It frequently takes place of animal and vegetable substances, forming petrifactions, or running, like lava or wax, into a mould, occasionally passing into the state of chalcedony, chchalon, &c. Fragments of this specimen became opaque in burning, as did that from Scotland. Crystals of specimens nearly dodecaëdral on Lancashire and Bristol iron ore are properly rock or mountain crystal, as they burn transparent.