I know this is an older discussion, but hoping someone can help out. I actually have 4 pieces of this glass/obsidian and have long wondered if it was truly natural or slag. It was found by a woman in the Mojave Desert near San Bernardino just like you said Joyce. She wouldn’t give an exact location because she wanted to keep it to herself and I respected that. Here are some photos to share and possibly answer some questions. I’m not a mineralogist or geologist, but what struck me as unusual about these specimens was that sand seems to be integrated throughout them, and you can see what appears to be christobalite spheres inside, something that one can sometimes find in natural obsidian. Excited to see if anyone can provide further observations and thoughts on these!