|North America||Lion Hill District, Ophir, Tooele County, Utah||16" x 9" x 7"|
As stated in the label, these are scalenohedral crystals. An ideal scalenohedron looks like the drawing to the right. As crystals grow, atoms may be added in a non-uniform way, or there may be physical obstructions that hinder growth in some directions, so many crystals do not appear quite ideal in shape. At the atomic level, though, the atoms are arranged properly.
Now, what about the "prominent twin" mentioned in the label? Twins are two or more crystals that are intergrown in such a way that one individual crystal is related to another individual crystal by some operation of symmetry that is not present in either one. The drawing below represents a twinned scalenohedron of calcite. The arrow indicates a "reentrant" angle - that is, a "concave" angle back into the crystal. Crystals don't grow this way - they always grow outward - so a re-entrant angle is a dead giveaway for twinning. In this case, a mirror reflection that is not present in the single crystal relates the two individuals.
Twinning is a very common phenomenon, and many different minerals exhibit some type of twinning. In fact, there are a few minerals whose crystals rarely occur untwinned.