here are my meager gemstuck contributions
Each of the characters can summon their associated strife weapon from their gem. John gets a hammer, Rose gets wands, Jade gets a rifle, Karkat gets a sickle… and so on. That one’s fairly obvious.
Gem placement is usually, but not always, governed by class. Knights’ gems are on their shoulders. (Dave’s gem cracked, and it summons a broken sword.) Seers and Witches get forehead gems. Sylphs have their gems located in the abdomen. Thieves have a gem in place of one eye, and Rogues have them in the palm of a hand (both relate to stealing: see what you want, take what you want). Princes and Maids have gems in their bosom/heart. Bards have crotch gems, a parellel to the legendary codpiece.
Breath players are one exception: John’s and Tavros’ gems are located on their backs. Both can summon wings in some way.
Role inversion results in a discolored, sort of clouded over gem. Witch of Void! Rose’s gem
and wands are black, Seer of Time! Jade’s gem changes color frequently and randomly (like her randomized shirt signs), Crocker! Jane’s becomes bright red… and so on.
(( edit: Also (I can’t believe I didn’t think of this sooner) inversion is the same thing as gem corruption. When Rose gets corrupted, she turns into an eldritch beast. Jade turns into something Light-based, possibly a big eyeball monster? Jane gets all techno-y/artificial. Aradia becomes a frog-like shapeshifter, Tavros’ form spouts red miles, and Gamzee’s is bliss- and lax-inducing. ))
John and Jade love to fuse, and have loads of fun doing it. Dave and Rose have a harder time fusing, especially because of Dave’s cracked gem; they tend to save their fusion for the direst of times. John fused with Rose once, but found it really weird. Ditto to Jade and Dave.
Other common fusions include Equius/Nepeta (so stable they spend as much time together as they do apart), Terezi/Vriska, Sollux/Karkat, Feferi/Sollux, Vriska/Tavros (usually Vriska pressures Tavros into it against his will — an abusive fusion derived from an abusive relationship that almost always results in disaster), Eridan/Sollux (extremely powerful but unwilling to act), Nepeta/Feferi, and Dirk/Equius.
Calliope’s gem is green and located on her left cheek; Caliborn’s is red and on his right cheek. Together they form a very powerful — and very unstable — gem fusion. They would have unfused a long time ago, but Gamzee thought it would be hilarious to use advanced gem technology to weld them together permanently. Now the two souls, unfused in all but physical form, vie for control over their powerful shared body.
Also Caliborn wants to fuse with Jake and overpower both of them.