You're a gem that was just wondering around when the Crystal Gems. Them being the Crystal Gems and seeing an unknown Gem assume you're a threat

Garnet: Has her Gauntlets equipped Who are you.

Pearl: Pulls her spear out of her gem Who are you!? Why are you here!?

Amethyst: Pulls her whip out of her Gem Woo! Fight!

Steven: Standing to the side Why do you have your weapons out? They haven't done anything...

Amethyst: Just wanting a fight she cracks her whip and whips it at you