The "Settlement" ability, amongst others, starts needing diamonds as a currency to purchase them after you have placed your first few. Unfortunately, I don't appear to know where to get more diamonds from!
I appear to have picked up a couple from completing certain cards, but in general, where can I get more diamonds from?
Best Answer
You need to find and learn "mining" to mine for gems, mainly. Gems are mined from the yellow brick road, as I call it (streak of yellow in the ground).
You also get gems from some cards, from some multiplayer.
In the game right now, don't waste gems on the settlements though. You will need them to get resource cards to attempt to get into the bronze age.
Update: In patch 1.3, multiple additional diamond spawns were added, making diamonds a much more available resource.