Shiny lump of rock

Any geologists in the house? I'd like to have been a bit more erudite with my caption but even after looking on the web I can't be sure what this rock is. I'm wondering if it's a type of obsidian.

It's been on a shelf for years. I can't remember when I bought it or where I bought it; and if it ever had a label it's gone. But I do know why I bought it after playing a light over it. Viewed front-on under light the rock is nothing special but if you angle a lamp sharply it fair glows - the kind of blue you see in peacock feathers.

I didn't venture out today and didn't get round to a blip until quite late in the evening and even then I was a bit stumped for inspiration. There's not much more to say about this except that it's only polished on one side. The underside is pretty dull. But I do like the shine and the glowing colour. It looks a bit blurry on top but the image is sharp. You can see that at the edges. It's just the way that it shimmers.