From the Sigma YouTube Channel:
Utilizing an advanced material known as “Clear Glass Ceramic”, the SIGMA WR CERAMIC PROTECTOR is a highly reliable filter with outstanding protective capabilities.
The shock resistance of the SIGMA WR CERAMIC PROTECTOR has undergone a rigorous drop ball test based on the ISO 12311:2013 standard using a 49g metal ball, 3 times the weight of a standard metal ball. The test proved the excellent shock resistance of SIGMA WR CERAMIC PROTECTOR compared to conventional protectors.
B&H carries the Sigma WR Ceramic Protector filters.