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Let's Rock => Rock Talk => Topic started by: Scalebar on April 21, 2019, 04:46:08 AM

Title: Kalahari picture stone
Post by: Scalebar on April 21, 2019, 04:46:08 AM

Anyone know much about this stuff? It really winds me up that it's invariably described as a jasper when it's obviously not - it's softer and it goes fizz if you drop HCl on it. My best guess is a dolomite, I wonder how such intricate banding formed. Anyway it works easily, takes a good polish and looks nice, plus I enjoy cutting it. Picked up a half kilo for a quid at a garden shop so I'm happy.

Blast, can't seem to attach pics - I'll blame the phone and try later

Title: Re: Kalahari picture stone
Post by: VegasJames on April 22, 2019, 12:07:39 AM

Sounds like calcite. Dolomite does not react to acid like that unless powdered. Calcite is much more reactive to acid.

Title: Re: Kalahari picture stone
Post by: Scalebar on April 22, 2019, 01:12:37 AM

The unground rock reacts slowly with hcl, ground much faster, little reaction with acetic acid. Dark bands are harder and spark a little with a dry cutting tool. It's a fun stone to work with

Title: Re: Kalahari picture stone
Post by: Orrum on April 22, 2019, 03:37:40 AM

Very nice landscape!

Title: Re: Kalahari picture stone
Post by: Scalebar on April 22, 2019, 04:15:49 AM

Thanks, the pattern ends up dictating the profile of the stone - I had to leave a very high dome to keep that 'mountain' on the 'horizon'. The back has a good pattern too so I'll have to look at open backed mounts for these.

Title: Re: Kalahari picture stone
Post by: Rustycat on April 22, 2019, 08:19:47 AM

Quote from: Scalebar on April 22, 2019, 04:15:49 AM

Thanks, the pattern ends up dictating the profile of the stone - I had to leave a very high dome to keep that 'mountain' on the 'horizon'. The back has a good pattern too so I'll have to look at open backed mounts for these.

I wonder if you have Rhyolite?

Title: Re: Kalahari picture stone
Post by: Enchantra on April 22, 2019, 09:49:50 AM

Looks like a banded limestone to me.  Limestone can have all sorts of patterns to it.  If it reacts with acid, it's definitely not jasper.
There are Limestones out of the Southwest US that have similar banding.