Stop being a tw at

Answer 1

The question assumes you're asking when do diamonds stop being described as total weight.

One description of a diamond is its carat weight. Individual stones each have a carat weight. When smaller stones are clustered or installed in jewelery, especially in an attempt to make the cluster appear like a larger, single stone, the cluster is often described as [diamonds] tw or total weight.

Diamonds stop being identified as tw when each diamond is weighed individually.