[Scott Hebert]

Generally speaking, it's easiest with Clams. Clams generate Astral Pearls, which for a 2-1 correspondence can be more of the gems used to make it. Someone better than me showed the slippery slope of Clam production, but here's a small analysis.

Say you start the game as Atlantis.

Turn 1:
Production: +5W
Make a Deep Seer.
EOT Gems: 5W
Turn 2:
Production: +5W
Make another Deep Seer.
EOT Gems: 10W
Turn 3:
Production: +5W
Make a third Deep Seer.
EOT Gems: 15W
Turn 4:
Production: +5W
Assuming you've hit Construction 2 by now, have 2 of your Deep Seers make Clams.
EOT Gems: 0W
Turn 5:
Production: +5W
Assign the 2 Clams to someone.
EOT Gems: 5W
Turn 6:
Production: +5W, +2S
Have a Deep Seer Forge a Clam.
EOT Gems: 2S
Turn 7:
Production: +5W, +2S
Assign the Clam to someone.
EOT Gems: 5W, 4S
Turn 8:
Production: +5W, +3S
Forge a Clam.
EOT Gems: 7S
Turn 9:
Production: +5W, +3S
Transmute your 10 pearls into 5 water gems, Forge a Clam, and assign your Clam to someone.
EOT Gems: 0
Turn 10:
Production: +5W, +4S
Assign your clam to someone.
EOT Gems: 5W,4S
Turn 11:
Production: +5W, +5S
Make a Clam.
EOT Gems: 9S
Turn 12:
Production: +5W, +5S
Assign your clam to someone, transmute 10 pearls to water gems, and forge another one.
EOT Gems: 4S
Turn 13:
Production: +5W, +6S
Assign your clam to someone, transmute 10 pearls to water gems, and forge another one.
EOT Gems: 0
Turn 14:
Production: +5W, +7S
Assign your clam to someone.
EOT Gems: 5W, 7S
Turn 15:
Production: +5W, +8S
Assign, forge.
EOT Gems: 15S
Turn 16:
Production: +5W, +9S
Assign, transmute, forge.
EOT Gems: 14S
Turn 17:
Production: +5W, +10S

At this point, you can forge a clam a turn. Soon, you'll be able to forge 2 a turn, and it snowballs even faster.

Note this assumes you find no sites to increase your gem income. If you do, you can transmute them to more water gems, and ramp up faster.

The other gem producers aren't so easy to exploit. The Fever Fetish diseases its owner, so basically you have to give it Undead to 'feed on'. OTOH, it does produce fire gems, which are needed in its construction. Transmuting 4:1 to get to Nature, though, is a killer.

Finally, the Blood Stone is rather hard-set, because you can't 'transmute' blood slaves.