Steven Universe - what WOULD happen?

Gems are no stranger to the concept of personalized summonables, although their Gem weapons are likely more of a learned technique like Hamon.
An interesting thing to point out is Mikitaka Hazekura's shape-shifting. Since he can't see stands, it's more of a stand-like ability he must have been born with as an alien.
Since Gems are essentially aliens and their only physical body is their actual gem, it could be assumed that their Hard-light body is also a stand-like ability their were created with, along with the shape-shifting abilities.

This would also mean that if the arrow pierces the actual gem, the gem gets a new ability as well as change it's appearance.
If the Hard-light body is pierced, the gem would gain requiem-like abilities.

"Tone down the Jojokes"

The more people complain, the more the Jojo fandom will make more out of spite.

That's just how it is.