How do I create a new weight type, specifically, CARAT.
Seems complicated. Seems like the weight units list, and units of measure list,
is closed, and
hardcoded here: \administrator\components\com_virtuemart\helpers\shopfunctions.php. Conversions are also limited to this closed list. Try to ask the Author.
The solution might be to create the new custom field "weight" for carats...