There are several tunnels leading straight to either gold or diamond ore. The tunnels always seem to stop when you've found the ore. Most of the tunnels are never lit.
Example of a diamond tunnel...
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fimg547.imageshack.us%2Fimg547%2F3672%2F20130217181022.png&hash=30b5287fc2596f12ad1c07bf87614952)
...with this at the end;
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fimg402.imageshack.us%2Fimg402%2F1498%2F20130217181152.png&hash=441afbaa06cfd814b5e2d66a979fe696)
The end of a gold tunnel;
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fimg803.imageshack.us%2Fimg803%2F5144%2F20130217181733.png&hash=d21e17eaf1b46c659ecd9bee2034d8c3)