• the lunar sea spire.

the lunar sea spire is associated with blue diamond, considering that pink diamond's pearl is able to remember it without being treated with decorum and it is saturated in her color. the lunar sea spire is filled with statues with multiple pairs of arms, a trait unique to fusions (as shown in the 'fusion' short). pearl even points out one such statue.

the lunar sea spire is supposedly kept intact with the moon goddess statue at the apex of the spire, but even whilst the statue is missing, it doesn't sink into the sea. it makes sense, then, that at the base there could be an ocean goddess statue (after all, the moon is only half of the spire--if there was only one statue, shouldn't it cover both aspects of the lunar sea spire?).

• in the answer, and as depicted in her moon mural, blue diamond appears to have more than two arms (in fact, it seems to be two sets of two).

this indicates, to me, that the statues are in blue's image, again indicating that she is a fusion of two different gem types. however...

• blue diamond doesn't appear to be open about her status as a fusion.

in fact, she seems to hide it even from her courtiers. for this endeavor to be feasible, her second gem must be easily hidden, since she exposes her heart gem. perhaps a teardrop cabochon on her back, hidden by her veil?

• blue diamond has very closely-held opinions on fusion.

she immediately goes on the attack to defend sapphire in what is, from her perspective, an unwanted and non-consensual fusion. she threatens to kill ruby for what she's done: this reaction seems incredibly overwrought in comparison to how everyone else reacts to garnet's first appearance. perhaps blue diamond knows something more about fusion than do other gems.