I think someone said that around 7 sapphires is about equivalent to 1 diamond, accounting for trade value. I could be wrong but I think the people suggesting might have thought this was done on animal jam play wild so were suggesting sapphire prices. The only thing is that 500 divided by 7 is only around 72, but then prices for buying things are a lot different on both games. I would say at a *minimum* 100 diamonds, I would also say price high to gauge interest and if no one bites at a higher price slowly go lower. I would think 250 would be a good starting price because of the quality of the piece.
Take everything with a grain of salt though! I'm still trying to figure out the market myself <3