This is aimed at alchemists, but you can always find an alchemist to do it for you. Tipping is always optional :P
Lets assume these prices:
Eternal Fire = 20g
Huge Citrine = 1g
Dark Jade = 1g
Cost to make Earthsiege diamond = 22g. AH Value = 40-50g (on my server at least). Profit = 18-28g a piece.
Elemental Air = 20g
Chalcedony = 2g
Bloodstone = 2g
Cost to make Skyflare diamond = 24g. AH Value = 30-35g. Profit = 6-11g a piece.
So, what I did was make 10 of each a day (so as to not flood the market) and sell them on the AH. Total profit for 10 of each would be 240-390g.
Now, I'd recommend being a Transmutation Master.
Making a gem I'd often get x2 Earthsiege/skyflare diamonds and sometimes x5. So imagine multiplying your profit by 5 occasionally for every 1 gem you want to make.
Don't forget transmuting an epic gem every 24hrs eitherthats an extra 120-200g.
IF YOU can get a JC to cut them, all the better. Check your servers AH to see which cut prismatic gem gives the highest profit and get them cut. You can double your gold like this.
Its easy money, and if you want you can try posting more at a time, but people might undercut you.