Most obsidian is black or a blackish-green, but this - as you can hopefully see - is a vivid deep blue which I have never seen before in a piece of volcanic glass. The blues and greens (I guess) must come from the copper or cobalt content of the ground that the molten lava flows over as the volcano erupts, and the natural glass is created by the silica sand fusing in the great heat.
This lump was most likely brought back from a Grand Tour of Italy and Greece that the grotto-builder embarked on sometime during the late 1700s, and is quite possibly from Vesuvius, which was a popular, smouldering destination at the time. There are many 18th century coins in existence, set in little lumps of set lava as mementos of their visits up the slopes of the volcano. It was fashionable to build grottoes to house private geological specimens to show to your friends, and even in 1790, only the very wealthy could afford to make them. The one I restored (at a cost of about £15,000) still has it's original bill for labour, and that bill was for about £40,000 - in 1790!
I didn't really steal this lump (honest) - it came from a pile of ordinary rubble set behind the face of the wall, and it wasn't until I was about to throw it away that I realised that it was not ordinary rock, so I polished it up to see how it looked.
A beautiful spin-off from another natural disaster.