Obsidian crystal from Paramore Crater?

As noted in the previous post, obsidian was found as inclusions in exposed basalt along the south rim of Paramore crater as well as scattered across the surface.  While examining and photographing the partially exposed nodule in the photograph below, an odd shape off to one side caught my attention.  An odd looking piece of obsidian with structure.  Obsidian is described as having no crystal structure, so why the tabular structure with several visible facets?  At first glance one might assume some altered rate of cooling resulting in a partial crystal structure but closer examination of the surface reveals a more likely explanation for the structure observed in this piece of obsidian.  In the second photograph notice the seam running down one of the facets.  This does not appear to be a crack but rather the junction between 2 masses of melted obsidian.  In addition, the surface is covered with the remnants of small air bubbles of uniform size, suggesting a small space between the surface of the cooling, and out gassing obsidian, and some other structure.  If exposed during the cooling process one would not expect a uniform surface of preserved escaping gas bubbles but rather a surface smooth surface as shown in the first photograph below.  The underside of the obsidian, a partial view in the third photograph, displays some bubbles but of varying sizes.  Rather than demonstrating some obsidian crystalline structure a more likely explanation is that this piece of obsidian represents a cast and is a positive naturally created from a negative in the host rock.

The next question that arises is more perplexing, if the crystal like structure observed in the obsidian is the result of a natural casting process that took place during a period volcanic activity in the San Bernardino Valley, what was the original positive that gave rise to the cast into which the molten obsidian migrated and filled.  The octahedral nature of the facets combined with the tabular layering found in a volcanic setting could be any number of minerals.  For example Pyrite or Spinel are both octahedral, but a diamond would also be nice.  Though I doubt the magma pipe goes deep enough to reach the temperatures and pressures required for diamond formation.

Obsidian nodule partially embedded in basalt

Upper surface of obsidian

End face of obsidian with larger air bubbles visible on the undersurface


  1. This is a very interesting piece of obsidian you found and then you explained this so nice to everyone.

  2. Thank you. It is my best guess as to why this particular piece of obsidian shows structure.

  3. Hmm, we were just saying the hiking group has a resident herpetologist, botanist, tree expert and more and that we needed a geologist. Sounds like we HAVE a geologist in YOU. Thank you!

  4. Thank you. I'm interested in geology, since I'm a longtime caver, but no formal training.

  5. Here is a reference to the geology and formation of the lava field including Paramore Crater.
    Petrogenesis of xenolith-bearing basalts from southeastern Arizona