Making stuff up here, but here’s a collection of my gem headcanons:
Topaz are sort of like messengers or enforcers for the diamonds
Jades are tacticians and military leaders, so they consult with sapphires and work closely with their diamonds (they're considered important gems, and so they tend to be pretty snobby as well)
Moonstones work on moon bases across the galaxies, and they’re often used as scouts or for recon (they’re usually the first point of contact with a planet the diamonds are looking to investigate/colonize)
Mother of pearls’ primary functions are to create personalized servant gems (generally pearls, but also perform customization functions on other “servant class” gems like jets, ivories, and corals if the client is a particularly high-ranking gem - think diamond/sapphire/jade level)
Pearls, while considered the fanciest of the servant class gems, are mostly there as a status symbol (pretty much canon). Others are available to provide a variety of services for high-ranking or heroic gems - jets are personal maintenance crews meant to stay in the shadows, corals are personal bodyguards (they're incredibly strong despite their small, cute size), ivories are personal messengers and provide reminder services similar to setting an alarm on your phone, and jadeites are fancy personal pilots for gems on the go (so think chauffeurs, while nephrites are more like military pilots). they can exist in a generic form as public servants in important diamond facilities (like the zoo), but those awarded to high-ranking gems can be customized by mother of pearls
opals are entertainers on homeworld for members of the diamond courts, but higher-ranking than pearls and tend to be regarded less as status symbols and more as minor celebrities (it’s an honor for an opal to perform at your function, so they’re generally reserved for the diamonds and their guests)
Serpentines are gem harvesters/bounty hunters that go after rogue gems and bring them back to homeworld for punishment (generally this involves being shattered or bubbled, like the rose quartzes, though can also lead to situations like lapis’ where they’re used to power items). they are generally feared on homeworld
Zircons are personal bodyguards to the diamonds and serve as body doubles if necessary (though this is rarely needed due to the power of the diamonds). their forms are tailored to mirror their diamonds, from outfit to gem shape and placement to personality and voice. because they have little purpose outside this function, all pink zircons were bubbled or shattered after pink diamond’s death as they were “no longer needed” and their forms were painful reminders. Yellow, blue, and white zircon are all still in circulation
melanites are soldiers less common/higher ranking than rubies but still below quartzes like jasper. they are still viewed as foot soldiers and are one of the first waves of soldiers deployed in battle
fluorites are scientists and technicians. they are the ones creating new gem technology and operating it (peridots are sent to monitor this technology and make sure it’s in working order, but are still below fluorites). they generally don’t see battle and are often placed in special r&d facilities by the diamonds
