The Ian Visits Blog has a very interesting article on what happened to the original, rejected version of the monolith from Stanley Kubrick's 2001: A Space Odyssey. He writes:
It may not look like it, but this 11 foot wide sculpture is the original model for the iconic Monolith used in the film, 2001 and it is on permanent display in St Katherine Docks, next to Tower Bridge. When Stanley Kubrick wanted a monolith for the making of 2001, he commissioned a local plastics firm, Stanley Plastics to cast the monolith out of a solid lump of transparent plastic. However Kubrick was disappointed with screen tests and the sparkling clear polymer block was eventually rejected as a prop in favour of the dense, black basalt that was imported from Scandinavia and is now such an icon of film history. ... That unwanted, and quite massive lump of perspex then sat in the Boreham Wood film studios until the Bratislavan born, London resident, sculptor Arthur Fleischmann acquired it. ... The block was kept in storage by Talbot Designs until Fleischmann received the commission to make a crystal crown for the Queen’s Silver Jubilee Celebrations in 1977. It was now that the transparent Monolith had the crown carved into its face and gained the perspective that we can see today. A dedicated open-air rotunda, known as the “Coronarium Chapel” was constructed, and the perspex block displayed within after being unveiled by The Queen.
To see more, better shots of the Crystal Crown, read the dedication plaque that describes the object's Kubrickean origins, and to find out where you can currently see it on public display, surf on over to Ian's blog. He's got you covered.