Jewelry is one of the easiest way to wash money.
There's really no way to appraise "the work" that went into a piece aside from raw stone prices.
Charge $100,000 for a chain that really costs $30,000, jewelers pocket some dough and kick back the rest of the $ to the customer back in cash under the table, etc.
Now I don't know if it's still going on but back in the days one of the largest corporate money laundering vehicles was advertising companies.
Mad corporate dudes created slush funds with kick back money from ad campaigns that supposedly cost X amount of Ms (cost for "creative direction") but really didn't.
Like, literally companies would compare kick back offers (on the low) from ad agencies when they did RFPs.
White collar scam game is on another level.
*edit: Art collecting world is also filled with scammers and slush fund creators. The same basic principle appiles here, too - arbitrary pricing system and diff. in book value and real cash transactions.