What is an avoirdupois ounce?
An avoirdupois ounce is the ordinary ounce of the imperial and US customary systems, as opposed to the troy ounce used for gold and silver.
It is equal to 1/16 of a pound or approximately 28.35 grams. The avoirdupois system is based on a pound of 16 ounces, with larger units such as tons and pounds used for heavier measurements.
What is a carat?
A metric carat is 200 milligrams (0.2 grams), the unit gemstones and pearls are weighed in. Karat with a K, which measures gold purity, is a different thing.