Getting diamonds as ftp is easy.
You would get:
-Daily arena: Let's say about 100, should be fair enough to expect you to rank in the top 100
-Tresore Cave: Ftp has 2 caves I think, so like a minimum of about 40 gems per day,plus plundering other also about 40 a day
So you would get about 5,4k diamonds per month.
If you buy 1 Touch of midas for your daily it would result to 5,1k
And if then you buy stamina twice a day, then slowly your diamonds would swind to
3,1k per month