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.
Topaz - Wooden model mounted on a hat
Based on a specimen from Shigar Valley, Pakistan.
I’ve been going to mineral shows with my achtaragdite crystal model handbag for a while now, and it’s been a reliable conversation starter every time. (If you haven’t read about my crystal model handbag yet, you can do so here.) People really seem to love the idea of wearable crystal model art, and after a little while I thought maybe the world needed another example - perhaps a hat this time! I went through my collection looking for a crystal with the correct proportions and the right level of complexity, eventually settling on a perfectly transparent little topaz from Pakistan.
Cumengite crystals - epitaxial over boleite
Based on a specimen from Amelia Mine, Boleo Dist., Baja California Sur, Mexico
This is one of the iconic shapes in mineralogy - highly symmetrical epitaxial overgrowths of cumengite over boleite. Although my specimen is tiny, its form is good, and finding it was all the encouragement I needed to get started on a model. This was the first model I attempted with nonconvex regions, and I was surprised by how well it worked out. Gluing these models together accurately is difficult, and sanding inside the concave areas is worse yet. But some of the most interesting crystals out there have concave areas, so developing a technique to represent them was clearly necessary!
Quartz - Right and Left Handed
Based on a specimen from Uri, Switzerland
I have spent a lot of time over the years digging through bins of cheap quartz crystals looking for right and left handed examples. After a lot of searching, I have a number of each, but most of those crystals are only singly terminated, damaged, or without a location. When I started making models, I decided I was in the market for a better specimen, and happened across this beautiful left handed doubly terminated crystal with textbook form at a show a few years ago. The models shown below are based on this specimen.
Calcite - Scalenohedron with Rhombohedron Faces
Based on a specimen from Joplin, MO
The calcite crystal this model is based on has an attractive combination of forms, with the added bonus of a sharp orange phantom inside. On the surface, the faces of the rhombohedron {02.1} bevel the edges of the scalenohedron {21.1}, while the phantom appears to be the scalenohedron alone. This model shows only the surface forms, although I am contemplating the future construction of a glass or plexiglass model with a visible phantom model suspended inside.
Tennantite - Tristetrahedron with Dodecahedron and Tetrahedron
Based on a specimen from Zacatecas, Mexico.
Most of the crystal models I make are chosen for the simple reason that I find the particular combination of crystal forms attractive. This little tennantite crystal is no exception, and as soon as I saw it under magnification I decided it needed a model that was easily visible to the naked eye. This one exhibits three crystal forms, a tristetrahedron, dodecahedron, and tetrahedron.
Bixbyite - cube and trapezohedron
Based on a specimen from Topaz Mountain, UT
This beautiful specimen of bixbyite on topaz was just crying out for a model! Nothing too complicated here, just a cube and trapezohedron in nicely balanced proportions. Cubes of bixbyite often have trapezohedral modifications on their corners, but often the modifying faces are tiny. This crystal has comparatively large trapezohedron faces, which makes an attractive combination of forms. The photos below are taken with the same orientation, looking down a 4-fold axis. The other photo was taken looking down a 3-fold axis, and really showcases the unique wood that makes up the trapezohedron faces.
Pyrite - Pseudoicosahedron
Based on a specimen from Bingham, UT
Some crystal habits are so distinctive they end up with their own names. This one is one of them, known as the pseudoicosahedron. It’s found in pyrite crystals when octahedral and pyritohedral faces occur in a particular proportion relative to each other. When the sizes of the two kinds of faces are exactly balanced, the 8 octahedron faces and 12 pyritohedron faces are all triangular, and are quite difficult to tell apart. The whole shape appears at first glance to be composed of 20 equilateral triangles, and resembles (but is not equivalent to) an icosohedron, the 20-sided Platonic solid with 5-fold symmetry.
Pyrite - Pyritohedron
Based on a specimen from Huanzala, Peru
One of the first things you learn in a crystallography class is that one, two, three, four, and six-fold symmetries are all possible, but a true 5-fold axis of rotation is impossible in crystals. (Objections relating to quasicrystals are noted, but the structure of quasicrystals is not periodic as required by the strict definition of a crystal. I will write a post on quasicrystals if/when I get my hands on a specimen!) Some crystals, however, appear at first glance to have this forbidden symmetry, although a careful check of their interfacial angles will prove that they do not.