Steinkaulenberg is the most direct way to understand Idar-Oberstein's stone trade. Instead of only seeing polished gems in cases, you go underground into the landscape that supplied the local cutters for centuries.
Tours explain how agate and other stones were mined and worked before imported material took over. It is slightly outside the centre, but worth the detour if the Felsenkirche gives you the town's silhouette and you want the substance behind it.