I always find the Webkinz Diamond in the top mine all the way to the left what ever it is called. For me it usually is within the first couple on either side. Good luck
The Webkinz diamond is in the Buried Bones Mine. You are not guaranteed to find it there, but that is the only place you will find it. Just keep searching. I have 2 crowns and am working on my 3rd and that is the only gem I have left for this one. Best of Luck.