I found this rather unusual rock/stone at Pwll Du Bay, South Gower yesterday. I say unusual as 99% of all the stones in this area are limestone. It's heavy for it's size and doesn't scratch, smooth on one side with a fine chopped matt strand fiber glass pattern in it. The sides are just beautifull with feather like veins of pyrite, fools gold? running through in various swirling patterns, the underside is a totally different texture like cracked open frozen chicken. I've also added a photo of a more normal stone for this area but love the quartz shaped rocking worm
The swirl is probably a cross-section of a gastropod.
The strange rock... Nice description.
rock 2.jpg looks as if it has been prepped by a fossil hunter.
rock 1. jpg and rock 3.jpg do indeed show interesting textures.
I think it is a lump of mineral, possibly an ore mineral, but I can't say which.
I'd need to thin-section it and have a look under the microscope to understand the textures and hopefully ID the mineral.
Otherwise, maybe somebody else actually recognizes it?...
The bay has the Black Rock Limestone subgroup nearby, comprising dark grey or black limestone.
As explained here, it has been partly dolomitized and can be very fossiliferous (especially with crinoids). I wonder if either of those factors may account for the macroscopic textures shown in your specimen (both could give rise to haphazard patches of coarsish crystal structure akin to what we see).
But that is only a very much half-baked idea; it does not explain the greater density, hardness and apparently complete absence of light minerals.
I can't find any other local even vaguely-relevant rocks on the map. But it could have been brought-in from elsewhere, of course.
I can't consult the Geol.Soc's page on The Gower - it is down for maintenance just now
There are various other sites dealing with the geology of The Gower Peninsula, but in none of them can I find anything to explain your rock.
Pwll Du Bay is one of Gower's best beaches and has a fantastic history. What is a little intriguing is some of the rocks at the back of the bay are from Cornwall. Ships from the West Country sailed to the bay and deposited their local rock ballast in lieu of the local limestone cargo which was taken back and used as fertilser. The headland shows signs of quarrying and the two buildings near the beach were Pubs.
Back to the rock in question. I've been wondering if the rock is actually a piece of slag. Perhaps due to it's density and general appearence. During the 17th-18th Century the nearby City of Swansea was the Copperolopis producing over 60% of the Worlds Copper. This process must have produced enormous amounts of slag yet rocks like these are very unusual finds and appear so unnatural. If it were slag would there be signs of more of a mixture of elements? I'm just guessing!
Slag comes in quite a few forms, but it often has signs of flow and often has vesicles (bubbles), neither of which are present in your specimen. However, massive slag is known - see the base of the sawn specimen at the end of this paper - http://www.arber.com.tr/imps2012.org/proceedingsebook/Abstract/absfilAbstractSubmissionFullContent326.pdf
So Cornish slag might be a possibility.
The frozen-chunk texture, howerver, is more suggestive of natural mineral aggregagation. If connected with copper (or other metal) mining, one could suspect the ore itself. But one would wonder why/how it escaped being smelted; just by accident perhaps. Alternatively, it could be gangue (unuseful) mineral associated with ore - which would be good material for ballast since it was loose, perhaps dense, and needed to be disposed of.
If it is a mineral aggregate, I still don't recognize that frozen-chunk texture.