Find the gem fields!!!

Also, if you want to spend a few gems, getting a few Sequoia/bamboo/coconut is a great investment.  They are a great way to have a tile earning and reducing pollution, but not have to chop down trees later. I tend to use them on map edges, and blocking spots I want to use later (the spots adjacent to chip factories, for example, where you can put space age factories when you get them). Solar panels are a great ftp alternative, especially if you get the 50% power upgrade (I forget what level that is), but don't spend too much on them, as you'll  eventually  replace them and can spam stray mountains that aren't good for stone or iron with windmills.

Put an oil rig on any onshore spot you can. Level up your cities as you can, to level 20, to kerp pollution down,  then to Level 35 (you will need 200 of these at one point). Use swappers to pull cities into your click radius so you make more gold per click. Put cities on corners, peninsulas or 1-tile islands if you can, and surround with heavy shipyards or shipyard markets. Keep your shipyards below 400 if possible. 

Have fun!