It depends. Some of them are probably 18k. some 14k, and some 10k. Real gold is too soft to make necklaces out of.
If you watch THE A TEAM, you will note that his necklaces get more numerous as the seasons continue.
24k gold necklaces were widely available in gold shops in the Chinese section of Bangkok the last time I was there. They don’t corrode or cause skin irritation. Obviously not meant to be worn during rough activities.