A Ruby amulet is made by using a gold bar on a furnace, with a ruby in the inventory. It requires a Crafting level of 50 or higher and provides 85 Crafting experience when made. The amulet must be strung before wearing or enchantment by using a ball of wool or the String Jewellery spell. Stringing provides 4 Crafting experience. A total of 174 Crafting experience is granted when cutting the ruby, making the amulet at a furnace and stringing the amulet.
Grum's Gold Exchange in Port Sarim buys unenchanted jewellery at a better price than High Alchemy, depending on his current stock. If he has no ruby amulets for sale, he will buy the first for 50% High Alchemy Value. It's a possible item required for a Treasure Trail emote clue.