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I'm currently writing an essay on a mosaic in the British Museum, and am unsure about which stone the frame is constructed from (pictured). I would be most grateful for help identifying it.

Thanks!

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    $\begingroup$ What mosaic? Maybe the answer is already 'out there'. $\endgroup$ – Jan Doggen Jan 9 at 13:32
  • $\begingroup$ The last picture reminds me of agate $\endgroup$ – Fred Jan 9 at 13:46
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    $\begingroup$ Hi, Fred, thanks for your input. I think it's more likely to be onyx than agate, as the bands / striations in onyx are parallel, whereas in agate they're curved. I think they're closer to being straight / parallel in this example. $\endgroup$ – Dylan Thomas Jan 9 at 15:34
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    $\begingroup$ Hi, @MyCarta, I think you may be right, and especially because I've just discovered (thanks to your suggestion) that there was an ancient Roman alabaster quarry very near to the site where this mosaic was found. I think that's my problem solved. Kudos to you, and thanks a lot! $\endgroup$ – Dylan Thomas Jan 9 at 16:19
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    $\begingroup$ Ah, how interesting. Yes, the quarry is Circeo in south-western Lazio. Link to the scholarship is here: oxrep.classics.ox.ac.uk/docs/Stone_Quarries_Database.pdf $\endgroup$ – Dylan Thomas Jan 9 at 16:24
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Summarizing into an answer some information and a recommendation provided in my comments.

Onyx and Agate, both Silicates, can be differentiated based on bands as suggested in an earlier comment: they are parallel and almost always perfectly straight in the former, curved in the latter.

However, this frame could be made of Alabaster (Gypsum) as well. Gypsum is a much softer mineral, with a hardness of about 2 in the Mohs scale (shown in the figure below, from this site), whereas Onyx and Agate being Silicates, should have hardness around 7 like Quartz.

Mohs Scale

A scratch test with the steel nail or steel knife would allow to discriminate between Silicates and Gypsum: if the knife scratched the mineral, then it would likely be Gypsum / Alabaster, whereas if the mineral scratched the knife surface it would likely be a Silicate, in which case the bands would be the discriminant between Onyx and Agathe.

I realize this being a mosaic frame in a museum you may not be able to do a scratch test with the steel knife, so I recommend the safe nail approach: try to rub your fingernail against one of the corners or edges of the frame: if the mineral does not scratch your nail, then it must have a hardness lower than 2.5, hence excluding Silicates. You could then assume the frame is made of Gypsum/Alabaster; if it does scratch your nail, then it must be harder than Gypsum and you can assume it is a Silicate.

NB a further assumption made here is that the mineral is not Calcite or other Carbonate; this is based on aspect only, since it is unlikely a test with hydrochloric acid can be done.

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