So I just thought of a cool little tactic, I don't know if anyone has thought of this.
So Tom costs 15 diamonds per day right? Well that means you'll use him at least 7 times per day at least, possiblyup to 9. Well, if you keep calling shovels (and I seem to get about 7 from Tom every time). then you could use about 49-63 shovels per day in the mine. by doing so, not only are you getting a lot of ores, but diamonds as well. I tested this theory the other day, and I got 16 diamonds from 56 shovels (8 uses of Tom). That paid off for Tom and I got a lot of ore as well. You could sell the free ore you're getting or you could use the ore for achievements.
I find it to be a very plausible idea. You get plenty of diamonds with shovels. Worst case, you lose 3 or so diamonds to Tom every time, and you're still accomplishing achievements and making coin.
Just an idea I thought I would throw out there for everyone. =)