The Hampstead Observatory is one of the few observatories in London open to the public for stargazing, and weather permitting, it is reopening this weekend. The observatory's main telescope sates from 1923 and ideal for viewing the Moon, planets and double stars. As long as the skies are clear, you might get to see Saturn's rings, Jupiter's cloud belts, the craters and mountains on the Moon and some of the brighter 'deep sky' objects - star clusters, nebulae and the odd galaxy.
The diamonds formed under extreme pressure and heat deep in the Earth's mantle. If you find one, it will most likely look like a metallic or glassy pebble rather than a sparkly cut gem that you might picture in your mind. The volcanic soil also contains amethyst, garnet, jasper, agate, and various types of quartz (and you can keep those, too).