The rock is called Witwatersrand ('white water rock' - there's a waterfall that runs over part of it), and it's in South Africa. Roughly 50% of the gold in use today was refined from this rock. It also contains large amounts of uranium (in a compound called uranite) and several other hard-to-find elements of varying importance.
I saw a slab about three centimeters thick with two straight edges about 25 centimeters long each; the other two edges were irregular. It looks exactly as one would expect for a rock formed from fusing pebbles together: glittering yellow metallic pebbles, smallish circular white pebbles, imbedded in a smoke-texture quartz. It had been polished on one side, and the curling smoke texture and circular pebble cross sections were very clear. The other side was more irregular and a somewhat uninteresting mash of weathered gray.
The reason I got to meet this rock has a lot more to do with its scientific importance. Some of the sulphides that make up the rock cannot form in an atmosphere that contains oxygen or water. Hence, when it was first analyzed, geophysicists realized that the atmosphere of Earth must not have originally contained oxygen, which is an important realization. The rock was part of an unusually fascinating lecture for my astrobiology class (on why we're sure there wasn't originally oxygen in the atmosphere and oceans, and how it might have gotten there).
But hey, I got to touch a piece of one of the most important rocks in the world!