You might convince them that their diamonds are worthless, and that a better investment is cryptocurrency!
Mechanical watches, perhaps.
AFAIU by the time of British colonisation in the Carribean / West Indies, sugar was cheap and afforded plentiful calories (and caries) to the working class.
Along with (relatively) inexpensive tea, the practice of serving boiled water-sugar solution greatly improved health (given the lack of water treatment at the time), reduced alcohol consumption, and provided additional food energy. And that was by the mid-to-late 18th century so far as I'm aware.
I'm not aware of any time or place where sugar was considered a luxury item, at least not for any substantial duration (say, excepting famine, economic recession, or war).
no guy ever wants to shell out $$$ for a useless rock
I can ransom hospitals with cyber attacks and get paid millions sitting on North Korea/rusia/Iran and dont have to stand up from my desk.
I can run distributed anonymized illicit drug distribution risk free and get paid a lot.
I can hire hitmen to gather intel and harm my adversaries and arrange kinetic campaigns.
A lot of good use cases.