I do know: carets are for gems
It's spelled carat, with an "a", and it IS for gems. It's also
standard metric, with five carats per gram, so a carat is 200
and karets is for gold (gawd I hope this is right!)
Again, two "a"s, as in karat. and yes, it's gold. It is NOT a unit
of weight, however, but a sort of percentage, expressed not as 100ths
(percentage), but as 24ths.
For the others:
Grams are universal metric measurements, used for many things,
including sometimes metal. More by craftspeople than sellers of raw
metal, who use troy ounces. 1000 grams equals a kilogram, etc, etc.
The gram weight unit can be useful with small scales by dieters and
diabetics calculating calories or grams of carbohydrate, and by drug
dealers selling things they shouldn't.
Ounces, or ounces avoirdupois is the standard english ounce used in
common commerce. It converts to metric as, if I recall it right,
28.8 grams. This is the unit you find on cartons and cans in the
grocery store in the U.S. Sixteen avoir. ounces equals a standard
pound, which is a very small lightweight little unit of measure,
which is why there are so goddam many of them indicated when I step
on the bathroom scale. it's not that I'm overweight, its that pounds
are so light. right?
Troy ounces is confusing in that it also is an ounce, but this ounce
converts to metric at 31.1 grams per ounce. It's only use is in the
weighing of precious metals. A troy pound is even more confusing
since it's only 12 troy ounces, not the 16 we'd have in standard
english ounces and poinds. A troy ounce breaks down to 20
pennyweight, and a pennyweight breaks down to 24 grains. Most of the
time, one uses only ounces and pennyweights (dwt), pounds and grains
are less commonly used.
Some scales also measure pearl weights, yet another unrelated unit
of measure, labeled in the unlikely unit of mommes, which itself
breaks down, if I remember it right, into grains. unsure about that
part though. (I can look it up if you wish). It may be that pearl
grains are troy weight, not some fraction of mommes. The coolest part
of "mommes" for me is that some pearl dealers I've known prominantly
pronounce the final "e", making it sound like they're weighing
pearls in units named after their mommy. Always brings a slight grin.
Don't know why. Might actually be the correct pronounciation even.
Again, I don't know if that's the case...
Hope that helps.