My necklace says both 18K and 750.
However it has both a lovely twisted gold chain with freshwater pearls every inch apart along the gold chain.
How do you get a weight when it is this way?
Best you can probably do w/o removing the pearls is estimate. Measure the diameter & search for a weight range. If you were selling for scrap you could let the buyer make their own estimate (and probably expect the low one).