Opals or glass

I'm still finding interesting rocks going through the inventory. This little batch was in with some Amazonite and Chrysocola.

At first glance, I thought they were broken shards of slag glass. The shears are much shinier than any stone; even opal

I moved the piece from the far left and propped it up so you can see how well the light reflects on the shear side of that piece. Again I was pretty convinced that these were broken glass. But as I was positioning the one nugget, I noticed something odd.

These are dendrites; small micro-crystals usually found between layers of sedimentary rock. They are leaf shaped and often confused as fossils. The blurred ones are embedded in the specimen, which means they formed as the stone was forming.

Because dendrites are delicate, they would not withstand the heat of molten glass, therefore could not form. Dendrites have been found in opals, although it is uncertain how they formed, as opals aren't layered.

I'm tempted to take one these home...but I can't keep doing that :-(