Hey dude, Although I don't play in the F.D world, getting diamond (annoying mostly) but not as hard as you think, I am actually trying to build one too but cant get the diamond together. The best way is to buy it using a bank card (but no-one likes spending money) you can earn diamond by doing lots of different things. You can do a lot of different surveys, although they sometimes don't work so you have to pay close attention to what you have to do. Just click the + sign next to the diamond in the top corner and click 'earn your diamond', earing them in quests is another one, but its like 1 every 20 guests or something so it takes a while. Or by spinning the wheel in the summer event. All of these I continue to do and I'm on about 366 diamond so far. The other thing you can do is inviting a friend to FoE by sending them an email or the 'invitation link'
Hope this helps dude.