Diamond
If I were Munroe, I would have gotten some landmarks so I could triangulate my position on a return visit and find that diamond. After all, no killer apes to worry about any more. If that size a diamond really was flawless, it would be worth SO MUCH.
$200,000 - $999,999
Diamonds in this price range tend to be greater than 3-carats (though not always) and will require a customized setting (standard settings generally accommodate up to three carats).
http://www.adiamondbuyingguide.com/diamondpriceguide.html#200000
3 carat diamonds are 9.4mm in diameter.
http://www.diamond.com/wp-content/uploads/2012/03/carat_weightscale.jp g
That diamond was about 4-5 inches(100-125mm) in diameter. Thats roughly 31 carats.
Also, once the lava hardened, they could fund a very profitable expedition to mine up the Zinj diamond geode. Unless lava destroys diamond, but I don't think its hot enough(I'm no expert, I could be totally wrong). share