I'm thinking of a story wherein a short rogue-like man (probably not a dwarf, but perhaps some kind of halfling or a human little person) is contracted or coerced by a powerful wizard to track down a large, many-faceted crystal. The story follows this guy on his journey, and at the end, the short rogue/bard scratches one of the facets on the crystal before giving it to the wizard. This causes the crystal to explode when the wizard tries to use it in a spell, which kills the wizard.
During the story, the short rogue-like person comes across the siege of a castle or fortified town, and somehow he is conscripted (or volunteers) to help with the siege. I believe there's an exchange where he lies about being trained as a siege engineer (he's a rogue, remember). During the battle, the siege engines manage to breach one of the walls of the castle.
There may also be something with the wizard's daughter, or the king's daughter (whom the wizard works for). It might be that the short rogue-like person was caught with this daughter, and this is how he was coerced into tracking down the crystal.
This book was written in English and I read it perhaps fifteen years ago, definitely not less than ten years ago now.