Not sure if you can make them or whatever but. During the murder mystery quest i found a way to keep the necklace you get during. Not sure if its a bug or not but you cant trade the item and stuff o_O
I use to do this with The "Shield of Arrav" and made money selling them. I would get the phoenix halves, he got the black arm halves, and then i sold them. I made like 700K, but then it was patched with the varrock graphic update in 2 days.
All i had to do is keep them all with me, drop the ones i had, then keep getting it and dropping it over and over till i decided to pick them up. now they aren't trade able. :/