WARNING: SPOILERS FOR THE UPCOMING FANFIC. HIGHLY RECOMMENDED THAT YOU WAIT UNTIL I ACTUALLY GET AROUND TO WRITING IT TO READ THIS.
Update: the fic exists. This still has spoilers.
Note: Gems of the same type cannot fuse due to the specifics of gem placement.
Tier 0 Fusion (two individual humans)
There are no tier 0 fusions, as humans cannot fuse.
Tier 0.25 Fusion (one individual human and one individual hybrid)
Stevonnie (Steven and Connie)
Tier 0.5 Fusion (one individual human and one individual Gem, or two hybrids)
There are no tier 0.5 fusions, as Gems cannot fuse with humans unless they contain some Gem DNA, and currently only one hybrid is known.
Tier 0.75 Fusion (one individual hybrid and one individual Gem)
Pearleven (Pearl and Steven)
Tier 1 Fusion (two individual Gems)
Garnet (Ruby and Sapphire)
Opal (Pearl and Amethyst)
Rainbow Quartz (Pearl and Rose)
Malachite (Lapis and Jasper)
Pink Sapphire (Rose and Sapphire)
Red Quartz (Rose and Ruby)
Rhodocrosite (Rose and Amethyst)
Turquoise (Lapis and Peridot)
Azurite (Lapis and Amethyst)
Brown Diamond (Red Diamond and Green Diamond)
Dioptase (Diopside and Emerald)
Tier 2 Fusion (three individual Gems, or one tier 1 fusion and one individual Gem)
Sugilite (Garnet and Amethyst)
Sardonyx (Garnet and Pearl)
Carnelian (Garnet and Rose)
Tier 3 Fusion (four individual Gems, one tier 1 fusion and two individual Gems, two tier 1 fusions, or one tier 2 fusion and one individual Gem)
Alexandrite (Garnet, Amethyst, and Pearl)
Cluster (Amazonite, Heliodor, Red Beryl, and Zircon)
Tier 4 Fusion (five individual Gems, one tier 1 fusion and three individual Gems, two tier 1 fusions and an individual Gem, one tier 2 fusion and two individual Gems, one tier 2 fusion and one tier 1 fusion, or one tier 3 fusion and an individual Gem)
Agate (Garnet, Amethyst, Rose, and Pearl)