Science Questions

What rock looks like glass inside and mineral outside?

Sun, 15th Jun 2008

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Cliff Taylor asked:

I found a crystal on a beach that looks like glass where itís broken but it looks like a mineral on the outside. We can cut glass with it and it doesnít affect the stone crystal. Can only a diamond cut glass?


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

Harder materials scratch softer ones, hardness is measured on te Mohr 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 is another possibility  for your material.


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cliff taylor asked the Naked Scientists: I found a crystal on a beach that looks like glass at where it is broke but has a mineral look on the sides that are not broke but I can cut glass with it but does not effect the stone. What do you think? cliff taylor, Sun, 15th Jun 2008

Quartz perhaps?
Bass, Mon, 16th Jun 2008

What about obsidian (volcanic glass)? Where does that sit on Moh's scale? chris, Wed, 18th Jun 2008

RD, Wed, 18th Jun 2008

crrrrrrrrrrrrrap bill, Thu, 30th Jul 2015

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