FMF - Friends of Minerals Forum, discussion and message board

[RicardoM]

Hi, I would like to know the correct name for this rock, for a time we are making lapidary with this material but still do not have the correct name.

Here people known it as chalcedony but I am in doubt.

Is harder than glass (scratch it), but softer than quartz, found along rhyolitic rocks in Queretaro state, México.
Vitreous luster, and irregular fracture.

Help is much appreciated.

Thanks in advance.

Ricardo

[SteveB]

I’m thinking Gneiss, thats a guess NOT an identification

[alfredo]

Looks like obsidian, and the locality fits too.

[Matt_Zukowski]

I agree with Alfredo - it looks like a layered volcanic with obsidian and pumice.

[Bob Carnein]

I'd call it a welded crystal-vitric tuff--essentially what Matt said.

[James Catmur]

I never collected that sort of material near Queretaro, but obsidian and tuff make sense. I suspect 'chalcedony' is a way of not saying 'obsidian'!

[Amir Akhavan]

Obsidian with narrow bands of intense devitrification. The small dots are spots where the glass devitrified ("de-glassed", crystallized).