He broke it with bare hands...
I'm starting to think that this artist newer saw the original LN text. Someone just prepared for him... kind of short retelling with omitted details.
Indeed, Roland broke the chain in the LN. But that chain was feeble and copper or so at most. It was an accessory to hold the pricey stone.
I can also assume that it was probably meant for nobles to wear. No one would keep such an expensive stone in this rural area just for prisoners.