I had a technique that meant for a couple of months I found a gem virtually every day. When it stopped working I had to modify it . And yet again I am having a great deal of success .
In any mine, go to the very end (on either side) and click on the first rock. Then go back towards the center and click on every other rock. This usually results in finding a gem.
My previous technique was to start with the second to last rock, then click on every other rock heading back towards the center.
I found that the top of the cave works better; just because I'd sometimes run out of rocks along the bottom.
I also have found that if I find a really great gem I immediately log into my nieces account and find that same gem in the same spot for her.
I hope these tips work for other members. Especially with the appearance of Doug the Dog- every gem helps . Happy Hunting !