I would buy it at the grand exchange in varrock or in Al Kharid. They are very cheap. You use gold bars on it. Then a gem piece. Then you could enchant it with a spell.
a gold bar and an amulet mold will be needed but you can get one for free at lumbridge F2P
I personally advise you not to make onyx jewelry because you lose over 2M making it, but buy uncut onyx in the Grand Exchange, or buy trade and cut it. Note that there is a chance that you will smash and you lose tons of money. then get an amulet mold and smith it.
Im assumeing your talking about the quest that you need the key.To do this just buy a key mould,it tells you where to...then get some clay and use it on the mold. Thats really the only use molding clay has in runescape.
You must do the dwarven cannon quest, so you can buy the cannonball mold. Then all you need is the mold and steel bars. Which you then smelt together.
You take a sickle mold and bring a silver bar and use them on furnace.
You can buy soft clay on ge, and you can buy a key mold.
Amulets of magic can be made by using a gold bar with a furnace while having a cut sapphire and an amulet mould in your inventory (requires 24 crafting). Then collect some wool from a sheep using some shears, and spin it on a spinning wheel. Add the ball of wool to the amulet. This then need enchanting by using a cosmic and water rune (requires 7 magic) to finally create an amulet of magic.
A "holy mould" can be purchase from a crafting store, the grand exchange or traded from another player.
Either make one using a silver bar and a tiara mold or just buy one.
You can do this by a number of ways, first off get a high smithing and crafting level ( anywhere between 40-60 is good) so you can smith the gold into golden bars, then get a mold of a ring or necklace or amulet and go to a furnace and click on the bar and then click on the furnace and it should pop up as (Craft BLANK)use a mold with a furnace and have gold in your inventory, if you want to add gems have the gems too.. it will bring up a screen showing what you can and cannot make with what you have
in oreder for you to smith gold first you will need to get a mold and gold bars then go to a fernace in runescape and smelt it with the mould hope this helped if you have any questions e-mail me at angelobigboss@Live.com or add my runescape account (CYCLON3X111) or (illdog9512)
you get gold bars and a ring mold and a sapphire go to a furnes and click sapphire ring