So I'm making a fighter that used a large glass cube on a chain as his weapon but me and my DM are both new and dont know what "weapon" this would be considered for the handbook can anyone help?

Basically, all that matters are the values listed in the weapon entries. Choose whichever one feels appropriate and declare it to be that.

If it's one handed with 5ft range, call it a flail.

If it's heavy, call it a maul.

If it has reach, call it a whip, but change the damage type to bludgeoning.

If none of the options feel right, then you may need to homebrew something new.

thanks that helps a lot

I mean, is that really a flavored weapon? It sounds like a flail with a specific shape.