Dig straight down and you will find two diamond blocks. Dig around the two diamonds blocks and you find two entrance of the cave ( both of them is lead you to the same cave ).
But one of them you leading you to the emeralds first and another you leading you to the diamonds first. Digging around the entrance and all the blocks you has found you across everything else: iron, gold, coal and lapis. Maybe it take a lots of time, but anyway that is worthy prize.