Santa Barbara Museum of Natural History

I know that garnets come in several different colors but I'm not too sure about all of the different shapes these stones are found in. You can pick most of these up with a typical magnet. 

Ken, Ventura - September 21, 2020

Curator Response

Hi Ken,

Thanks for sharing these with us. The red ones sure look like garnets, but as many geologists will tell you, it’s never satisfactory to identify rocks, minerals, fossils, etc. from a picture alone. Without having an electron microprobe machine attached to my eyes (wouldn’t that be convenient?) it’s impossible to determine the exact composition.

As you alluded, garnet crystals come in a variety of shapes and colors. In general, all different types of garnets (with differing chemical compositions) can have the same shapes. These particular ones look like they have a trapezohedral shape (meaning the faces have at least one pair of opposite sites that are parallel to one another).

When determining what kind of garnet you have, color and magnetism are more diagnostic than shape. These might be almandine garnets; those are rich in iron, so it makes sense that you'd be able to pick them up with a magnet. However, the color in the photo looks like more of a magnesium-rich composition (like pyrope garnets) rather than one rich in iron like almandine, but the magnesium-rich variety would not be very magnetic, so it's a bit of a mystery!

This gem auction site shares a fairly non-technical explanation of some of the common shapes, including the trapezohedral shape: gemrockauctions.com/learn/technical-information-on-gemstones/crystal-shape-of-garnet

Stay curious,

Nature Education Manager Sabina Thomas, Ph.D.