I recently discovered a secret weapon you can get for Crono by opening one of those black boxes, and it is called a Swallow.
I'm sure you all know that Serge weilds a numerous amount of Swallows in Cross. But I have a theory:
The black treasure boxes are branded with the insignia of Zeal, which you open with The Pendant.
However, you cannot actually open the boxes until Schala dissapeared, correct? No one knows where Schala went, and It isn't explained until Chrono Cross when she was saved by Serge and fused with Kid.
Perhaps seeing the events of Chrono Cross, Schala traveled to the past sometime after CC and placed various sealed boxes in every era in order to Help Crono and the rest of the party on their quest. And one would be Serge's Swallow.
But why then would it be the second most powerful weapon in the game? one answer: It's the Mastermune.
The Mastermune is the second most powerful weapon you can use in Cross, right below the Spectra Swallow. This is the same with "Crono's" Swalloe, right under the Rainbow Sword which is also forged from Rainbow Shell like the Spectra Swallow. However, after all that hard work defeating Dario, you can't take it into a new game plus.
Why? Simply because Schala (the fused being) takes the Mastermune to give it to Crono.
But that doesn't explain why the Swallow in Trigger was the shape of a Katana. My conclusion to this is that Schala reformed it in order to fit Croo's fighting style.
But what doe severyone else think?