While never specific used, I do believe that Keyblade’s are just weapons forged of light (darkness if from the realm of darkness, or of heart if it’s the Keyblade of People’s Hearts).

I believe they are just magically summoned weapons like the others we see in the series, just stronger and more specific from those with powerful hearts and a little magic.

It’s no surprise that they can break. Everything can break. It’s never said to be unbreakable. Riku’s Keyblade breaks because darkness is far more powerful in its own realm. Mickey’s did not. He simply fused his two keyblades together via magic. Without a doubt, they should have showed that onscreen.

The reason I think we never saw a Keyblade break before (outside of Data Sora’s) is because it never really called for it.