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Quartz - Dauphine twin
Based on a specimen from Uri, Switzerland
The Dauphine twin law is a parallel axis twin combining either two left or two right handed quartz crystals. Quartz is commonly twinned according to the Dauphine twin law, but specimens exhibiting ideal form are less common than might be expected. Faces that would reveal twinning are not always expressed, and twin boundaries are often irregular and hard to see. That means in a crystal whose faces don’t reveal its handedness, twinned regions with seamless boundaries can pass unnoticed. (Models of quartz crystals whose handedness can be determined are shown here)
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Quartz - Brazil twin
Based on a specimen from Canton of Graubunden, Switzerland
The Brazil twin law is a parallel axis twin found in quartz, combining a right and a left handed individual. Ideal drawings of a fully developed, symmetrically twinned crystal are often found in textbooks, and make for some really lovely diagrams (see below for one of them). Unfortunately, actual crystals with these faces are extremely rare, but it’s possible that I have one in my collection. More about that below, but first here’s a wood model representing the ideal form, depicted in fig. 455 from Dana’s A Textbook of Mineralogy, 4th ed., edited by W. E. Ford.
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Pyrite - Iron Cross Twin
Based on a specimen from Gachala, Cundinamarca Dept., Columbia.
One of the most recognizable crystal drawings in any mineralogy textbook is the diagram of the pyrite iron cross twin. It’s a penetration twin of two pyritohedra, and their interaction creates the distinctive cross-shaped pattern where the edges of the two pyritohedra intersect at 90 degrees. Below is a drawing from a crystallography textbook, with the model I made next to it.
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Spinel - Spinel Law Twin Display
Based on a specimen from Luc Yen Mine, Yen Bai Province, Vietnam.
I love the idea of displaying a mineral inside a box or case that’s a matching crystal model. It seems like a perfect way to show off a specimen with particularly good form, and present an impressive or surprising specimen in a unique way. This twinned spinel specimen isn’t perfectly smooth or gemmy like some smaller crystals in my collection, but its large size made it the perfect candidate for a custom display.
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Staurolite Fairy Cross Twin
Based on a specimen from Semiostrovie (Sevev Islands), Russia.
When I was a kid, every attraction with any kind of geologic connection (museums, national parks, commercial caves) had a basket of staurolite fairy crosses for sale in the gift shop. Some of them were probably fakes, some of them were “improved” by a little judicious carving, and some of them had cheap findings glued to one end to turn them into pendants. I never bothered to buy one back then, because they seemed cheesy and incredibly common. Lately however, a well formed fairy cross is a little harder to come by. I’m keeping my eye out at mineral shows for a specimen that more closely resembles the form of the crosses I remember from my childhood, but this one from Russia will do nicely for now.