Here are a couple of important points to keep in mind when you are choosing a gold necklace.
Pure 24 carat gold is a soft metal and although looks stunning as a necklace would be far to soft for every use. Therefore most gold for jewellery is alloyed with copper or silver to make it more durable and harder and more long wearing. Hence 18 carat is better for gold necklaces and rings. One can get 12 and 9 carat gold and, although this is very durable, does not look quite so nice and, of course, has a much lesser value.
If one is looking to have gold for investment purposes then gold coins or gold bars are the way to go. If the time coms to sell your gold these would bring the best price. But gold jewellery would not bring such a good return and therefore is not something one would buy for investment purposes but purely to enhance the beauty of the body.
A gold necklace can look truly wonderful and bring out the best features one has so should really be used just for that purpose.