Board Thread:Questions and Answers/@comment-25549762-20141017161237/@comment-24145588-20141017213627

There are some things that can only be done with Diamonds, such as:
 * Upgrading the Castle to Extravagant or Paradise
 * Buying or upgrading to a Maximum Mine (assuming your game has it)
 * However, you need to have made a cash purchase at some point to get this
 * Permanently lighting a Wishing Torch
 * In my opinion, this is of no value, and extremely expensive
 * Umm... I'm pretty sure there was something else, but I can't remember right now!

You will pretty much need to get the Extravagant Castle at some point, if you want a lof of monsters, so that's a good use of Diamonds; no other way to get it.

If it's available to you, the Maximum Mine is a good idea, because it will very rapidly pay for itself: You put in the initial 20 Diamonds to upgrade it, and in about 3 weeks you're ahead of the game, since it produces an extra Diamond each day, when compared to the Mini-Mine. However, this would require that you spend some actual cash to start with, which you probably don't want to do.

Using your Diamonds for mundane things, like speed-ups or buying monsters isn't a good idea, since in that case you're just substituting Diamonds for a little patience. (I do occasionally use them for speed-ups, but maybe 3 or 4 a month, and I've got over 1600 saved up; when you have so few, it's a different story.)