The emerald necklace is made by using a gold bar on a furnace with an cut emerald and a necklace mould in your inventory. It requires a crafting level of 29 and provides 60 experience when made. Making this item will result in a profit of 119 coins.
Grum's Gold Exchange in Port Sarim no longer buys at a better price than High Alchemy. In the past, if he had no emerald necklaces for sale, he would buy the first for 997 coins but now it is the same 60% of the store selling price, 855 coins.