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This is a special rock to me. I've had it since the age of nine and I collected it at Sennen Cove, next to Land's End in Cornwall, when we lived nearby. There were boulders of it amongst the familiar granite. Strangely, I've never worked out before what it is (I'm fairly rubbish at rocks!). At the time, I assumed it was some sort of serpentine but you only get that miles away, on the Lizard. Anyway, some intensive internet searching has revealed its identity. This paper: Shallow laccolithic emplacement of the Land’s End and Tregonning granites It's spotted hornfels - which is metamorphosed shale/slate, cooked by the adjacent granite intrusion. Actually a relatively rare rock. The green/grey mineral is cordierite, present as quite nice pseudo-hexagons. It shows altered rims (pinitisation) which have weathered to give the red on the surface (I think). Cordierite is the mineral that Vikings used for navigation, to spot where the sun was in a cloudy sky. It's intensely dichroic - showing two colours according to orientation in polarised light. This has led to much more learning but I'll leave it there - this chunk of rock has taught me a lot! Submerged in water for colour contrast - about 9cm across Dry Closeup Polished section, submerged (scale 1cm) Pseudo-hexagon in centre (cordierite is orthorhombic), altered rim. Photographed submerged. From the linked paper that led to me identifying it (plate 8) - this is from the coast at St. Just, very near Sennen.

I found these all on the same hillside in Arizona, redwall limestone, Mooney member, Mississippian. I'm pretty sure 1 is a Hexactanellid sponge. The others I'm less sure of and would like to hear more experienced opinions on them. The second picture is just the backside. Thanks for looking. Bob @Arizona Chris

Hi, I found these two stones in a creek bed in Northern Missouri. The only two I was able to come across thus far. I've honestly never seen anything like this, and was wondering if any of you folks had some insight? Thanks!!! The bumps stick out on this, but they are smoothened bumps.