I found this rock by the river in northern California. It is very light, no traces of metal (doesn't stick to a magnet anyway). It's very glassy on the inside but dull on the outside. Any takers on what it is? I tried to find something similar to it on google, closest thing I could find was a lightning-struck rock called a fulgurite. Could this be a fulgurite, or just volcanic rock? I can see that it is very black in some parts, almost looks like fusion crust like a meteorite would have (but again, there's no iron/nickel in it). I appreciate any of your thoughts!