First let me say that I am no geologist, nor could I play one on TV. Having established my complete lack of credentials, I would offer that as a kid I collected and studied all kinds of rock formations, so the first thing that struck me was your reference to fire, which may be part of the answer. There are two types of natural glass; volcanic (the most common form being obsidian or "black glass"), and fulgurite which is the product of lightning striking the ground. The most common fulgurite examples you will see on the internet are the result of lightening strikes in sandy areas and they look nothing like this; however, there are many examples of fulgurite in different shapes and colors. Lightning strikes the ground at 3,000+ degrees F. Like Forrest Gump's box of chocolates, you never know what you're going to get. It all depends on the mineral content at the point of impact. It is super heated, then immediately cooled by the rain which causes the "splattered" look. Again, this is only one possible answer. I would take this to the closest university with a good geology department and have them examine it to get the final answer. Thanks for sharing!