Grandbows are recent creations, devised by Alethi engineers through the use of fabrial science, and each requires a small infused gemstone to maintain the strength of its pull without warping the metal.
Some call them Shardbows, but Adolin doesn't like the term. Unlike Shardblades and Shardplate - which are something special (i.e., relics from another time when the Knights Radiant had walked Roshar), fabrial science hasn't yet approached recreating them.
They are sleek and look dangerous.
* ... because Rock is ... just who he is.