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I find a great place to farm these are in the river in the Brisban Wildlands.

You will need a gathering tool to harvest the clams here. 

To narrow your search, it is in the south eastern part of Brisban Wildlands (a 15-25 area) - Use the Mrat Boru Waypoint and head north north east (about 1-2 o'clock). Clams seem to spawn in between the two bridges in the pool of water. The area is called Venlin Vale and the pool is just to the north of the area title on the map.

Ye I seen that on the wiki but havn't found any spots with clams!

Edit: Clams can be harvested like anything else that uses the 'Gathering' tool. They can be found at the bottom of lakes, and they are extremely small, but however do show on the map like everything else that can be harvested. They wont show on the map however until you are close to the bottom of the lake. I found the lakes in Kessex Hill's to be a good place to farm clams, with about 1 in 5 also giving a Pearl.

Something tells me these can be found harvesting clams...

Anyone have a good place to farm these. Unlike the Amber, Garnet, Turquoise, Malachite and Tiger's Eye's pebble's I don't seem to get these from mining ore's, gathering or lumbering down the tree's!?