Kid Icarus: Uprising has a StreetPass feature that allows you to trade your weapons to other players in gem form. There are three options in this menu: Create Gem, Fuse Gems, and View Gems.
Create Gem lets you sift through your catalogue of weapons and choose one to make into a Weapon Gem. You don’t lose the original weapon to do this, so feel free to switch your Weapon Gem frequently to trade among friends via StreetPass.
Fuse Gems works similarly to Fuse Weapons in the Solo mode Arms Altar, except that it costs you hearts to complete the fusion.
View Gems shows your entire inventory of Weapon Gems received via StreetPass. You can choose to convert a gem to an actual weapon, costing you hearts. You can also choose to convert a gem into hearts. New Weapon Gems can be received by tapping on the bag icon in this menu.