Remember when the rubies fussed to make one big ruby, with no physical shape difference? Well, Garnet proves gems can fuze for very extended periods of time, indefinitely it seems, what if the diamonds are just big fusions of their respective diamonds?
My understanding is because the triple Ruby fusion had three separate gems somewhere on her body and The Diamonds that have been shown all only have one gem in the center of their chest.
One would assume that ALL fusions all have multiple gems regardless if the gem is from the same facet or cut.