Same here, some diamonds went missing, but I finally found out where I spent it by accident. When MM starts up, there's a Shop window that sometimes greets you, right? Well, while MM is loading and all those screens appear (the magician, the scientist, the fortune teller) and sometimes you tend to tap tap tap to speed up loading but nothing happens. Actually something does happen - you're already tapping and buying stuff in the Shop. I bought those gold Cupid hearts by accident, two whole sets of them, on 2 separate instances, until I realized that my impatient tapping was actually buying stuff from the Shop and spending diamonds by mistake. So now, I try to be more patient when MM is loading and don't tap around the screen anymore.