What rock looks like glass inside and mineral outside?

15 June 2008

Question

I found a crystal on a beach that looks like glass where it’s broken but it looks like a mineral on the outside. We can cut glass with it and it doesn’t affect the stone crystal. Can only a diamond cut glass?

Answer

It certainly could be volcanic glass. There's a glass called obsidian, which is silicic magma. The way to recognise it is if it's got a fractured surface and it's curved, Glass generally forms these curvy planar surfaces when you break it.

Harder materials scratch softer ones, hardness is measured on the Moh scale: Diamond is 10, talc (of the powder) is 1. Window glass has a hardness of about 5.5, so there are a lot of materials which can scratch it, including hard steels (6.5), silica in quartz or flint (7) etc. In fact, flint is another possibility for your mineral as that can look very like a glass, so it's another possibility for your material.