not hallmarked

by sharon
(london,united kingdom)

why would a piece of gold jewellery not have a hall mark? could i sell it? if i need to hallmark it, how do i do this. it is a gold necklace, with jade, bought in hong kong in 1966 the jade stones are approx size of a 10 pence piece,but egg shaped. there are 6 stones set in gold with large gold links. the gold is not hallmarked.