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Thanks everyone. No x-ray diffractometer sadly (they sound fun!) but have tried Dirty Pete's suggestions. It's light and quite hard and when I put vinegar on it I couldn't see any bubbles. I'm not doubting that the two rocks could be from someone's collection but we've lived in this cottage for 25 years. Before then there was another family for several years but before that it had been a farm labourer's cottage since it was built and apart from WW1 they didn't go anywhere. The rocks were also found almost at the water table (about 18 inches down so not sure why they would have been buried so deep. I shall keep digging!
I've enrolled in a geology class in the summer and so hope to learn a little more.
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I have also dug this up in my garden (near Canterbury, Kent). I first thought it was man made marble or similar as the top surface is flat and there is a fairly straight small ridge along the side of the top. However on looking at it closely, the metallic (?) bottom (top picture) joins the different top material very unevenly and this looks natural. Would welcome any ideas as to what it may be please.  It was found in the same hole as the previous post I made.
P1030314.JPG P1030311_-_Copy.JPG P1030312.JPG 
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Dirty Pete
To get a better idea you'd need to do some tests, hardness/acid reaction etc, but from here I'd go for calcite and or dirty gypsum.

PS. Has someone thrown out their old rock collection?
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Thanks Pete,

Would these stones not normally be found together like this then and are they natural to this area (about 7 miles south west of Canterbury)? We've been in the cottage for 25 years so if someone got rid of a collection they would pre-date that and I found them about 18 inches down (nearly hit the very high water table!). My cottage has been here since 1839 and there was a previous house on the site before then so a lot of coming and going over the years. I do find broken clay pipes and lots of old china, including at least one piece which probably even pre-dated the old house as it was dated to medieval times.
I'm hoping to do a short U3A geology course in the summer as the subject really fascinates me.
Thanks again
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Could be baryte
Anywhere for little ammonites, twice as far for big ones!
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I agree with Pete, it looks like someone's old rock collection, not local to Kent.

It is very difficult to identify a mineral from a picture alone, but here are my thoughts on it:-

My first thought on the white mineral was calcite or barite.

Looking at picture 3 there are signs of cubic crystal structure, so then I thought fluorite.

 However, there are no signs of cleavage which would be expected on a piece that has been knocked about a bit.

There is an irregular crack on picture 2, so now I think it may be quartz that had grown on a cubic mineral such as fluorite.

If it will scratch easily with a knife point then it is not quartz, so it's either calcite, barite or fluorite.

I don't think it is gypsum, it is too soluble to last long buried in a garden.

I think the dark mineral may be oxidised/weathered siderite or possibly marcasite, both iron minerals, hence the rusty brown colour.


Roy D.
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Dirty Pete
Assuming you haven't got an X-ray diffractometer in the garage, other things you could try: barite is dense
(sp gr 4.5), pick up a similar sized local stone from the garden and compare them. Barite should be
noticeably heavier. Calcite will bubble with acid, 10% HCl is the norm, if you don't have any of that, scrape
some of the white coarse crystalline stuff into a glass using a knife, (If you can't scrape stuff off easily it's
probably quartz). Add vinegar and look very closely, you should see tiny bubbles emanating from the
scrapings. If in doubt putting the glass in the microwave for 5 seconds and heating up the vinegar will give
you a stronger reaction. 
Have fun
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