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This large polished boulder sits outside one entrance the University College Hospital in London:

UCLH boulder

Here is a close up:

UCLH rock close up

What sort of rock is it and how would it have formed?

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closed as off-topic by user12525, Fred, Erik, Jan Doggen, gansub Aug 27 at 7:49

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  • $\begingroup$ What's with the cover? $\endgroup$ – Mazura Aug 17 '15 at 13:41
  • $\begingroup$ @mazura it's a decorative light built into the Boulder. $\endgroup$ – matt_black Aug 17 '15 at 13:42
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This looks like a polymict (composed of fragments from different rock types) conglomerate, that has been polished. This rock is composed of many well rounded fragments of other rocks that have been eroded, transported and deposited in a new location. These fragments have then then undergone diagenesis (burial and compaction) to form the rock in the photograph. It is hard to tell for all the fragments from the photos but the fragments appear to be made from mostly metamorphic and some igneous rocks, such as gneiss and granite.

enter image description here

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  • $\begingroup$ Actually I think all of the clasts are metamorphic. They're either clearly banded or just don't look igneous. $\endgroup$ – Gimelist Aug 18 '15 at 13:15
  • $\begingroup$ I think there are a few small well rounded granites in there $\endgroup$ – Justin Aug 18 '15 at 13:44
  • $\begingroup$ Or migmatites... :) $\endgroup$ – Gimelist Aug 18 '15 at 13:57
  • $\begingroup$ Yup or possibly those :) $\endgroup$ – Justin Aug 18 '15 at 14:01
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The boulder is a sculpture; the rock type is almost certainly conglomerate, a kind of sedimentary rock.

Quoting here from an article about a geology walk, by Jean Gardner, in the winter 2014 newsletter of the Hertfordshire Geological Society:

[The] boulder is a conglomerate containing well rounded metamorphic and some igneous clasts ranging from pebbles to boulders in a fine crystalline matrix. There are a huge variety of clasts including schists, gneiss, mylonites, jasper, granites and serpentinites. These clasts indicate a continental collision and orogeny followed by erosion and fluvial rounding before deposition in a braided river or bar. It must then have been buried compacted with possible secondary metamorphism creating the crystalline matrix, be fore its final erosion. This means a history which predates the Andes and possibly starts with the Rodina continent of 1.2 billion years ago.

The rock is a bit unusual. As Robinson (2013) noted, it's very poorly sorted, and might even qualify as diamictite. It must be very well cemented to be polished so smooth while maintaining its integrity. The clasts are very well-rounded and up to boulder-sized. Lastly, its provenance seems unknown. If you Google 'Marinace conglomerate', you'll see some similar rocks (often wrongly called 'granite' by the building stone industry). I suppose it's possible the rock was man-made somehow, but this seems pretty unlikely.

Wherever the rock is from, I'm certain the boulder itself was polished, and probably shaped as well, by the sculptor.

As mentioned in another answer, it's a piece of sculpture at University College London Hosptial entitled Monolith and Shadow (2005) by former UCL Slade Professor John Aiken. Like most public art, it was a bit controversial.

Reference

Robinson, E (2013). The history of Monolith and Shadow. Geology Today 29 (5), September–October 2013, p 168–169. DOI 10.1111/gto.12019.

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Quoting from mlra66 Flickr page

Monolith and Shadow by John Aiken

University College Hospital, Euston Road, London.

The monolith is made from a recently discovered Brazilian granite - a rich and exotic stone that combines many colours, shapes and patterns. The decorative elements represent minerals ranging from granite, flint and quartz to precious and semi-precious stones and are embedded in a green sand base. The stone is the product of a prehistoric pebble beach that fused under intense heat and pressure millions of years ago. The monolith has been polished to a mirror-like surface to reveal the exotic composition of the granite.

It looks like the same material is also used in kitchen tops here where it is listed under alternative names Black Mosaic Gold Granite, Marinace Granite, Aquarium Granite, Nero Marinace Granite, and Black Morgan Granite.

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    $\begingroup$ I don't think the artist who made the block or the companies that sell kitchen tops are using very accurate geological terminology when they say "granite". It is useful to know what they call it, but I'm more interested to know what a geologist would call it not least because the salesmen seem to call everything granite. $\endgroup$ – matt_black Aug 17 '15 at 11:42
  • $\begingroup$ Polished stone suppliers are notoriously imprecise about the rock's nomenclature. There are 'granites' on the market which are actually granulite, diorite, syenite, norite, gabbro and various metamorphic rocks. $\endgroup$ – Gordon Stanger Oct 2 '15 at 13:53
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This shaped and polished boulder is a notorious 'work of art' costing 70,000 UK pounds! Officially, its' provenance is 'Brazil', with no further elaboration. My own best guess, based upon the varied lithologies of the boulders and matrix, is that it most likely comes from the Rio Itapicuru greenstone belt (a greenschist facies metamorphic belt) which is known to be associated with 'basement gneisses' and granitoids, which are such a prominant feature of the boulder.

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