Both ct and ctw refer to the metric carat, or 1/5th of a gram. The
difference is the ct normally applies to a single stone, IE A
2.67ct ruby. The ctw or tcw refers to the total weight of multiple
stones. IE a ring the has six accents diamonds for “1.20ctw”. This
means the there were six stones with an average ct wt of 0.20ct in
size in the ring for the 1.20ctw. It could also apply to a parcel
of loose stones, Don