This large polished boulder sits outside one entrance the University College Hospital in London:
Here is a close up:
What sort of rock is it and how would it have formed?
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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.
The boulder is a sculpture; the rock type is almost certainly conglomerate, a kind of sedimentary rock.
[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.
Robinson, E (2013). The history of Monolith and Shadow. Geology Today 29 (5), September–October 2013, p 168–169. DOI 10.1111/gto.12019.
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.
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.