The Core nut is a brass-infused synthetic matrix. They use bone on the Core 594s. When you drop the black Core nut on a quartz countertop, it has a very distinct, glassy 'ping'. Clear and pure. Paul has put out a video or two where he compares nuts using this 'ping' test. The plastic SEs nuts are fairly dull. The GraphTech TUSQ and TUSQ-XL ring pretty close to the PRS Core nuts, but I though the core nuts were clearer, glassier.