The onyx bracelet is made by using a gold bar, an onyx, and a bracelet mould on a furnace a with Crafting level of 84, earning 125 experience. It is the bracelet of the second highest gem quality in the game.
It is 1,877,626 coins cheaper in the Grand Exchange than the onyx and gold bar needed to make it; however, there are virtually none traded, since it is unlikely that anyone would make an onyx bracelet (because of this huge loss).
A player enchanting an onyx bracelet.
The bracelet has no bonuses of any kind until enchanted. If it is enchanted using the level 87 spell Enchant Level 6 Jewellery (earning 97 Magic experience), it becomes a Regen bracelet, which increases Life point regeneration to 3x the normal rate.