I’m pretty sure almost all the Sigmas are AlOx. I think one of the coarser ones are SiC…. Just checked. In their ceramic series only the 120 and 240 grit are SiC. The rest are AlOx. Not sure about their Select II series.
Ah right, yeah I wasn't certain about that part. Good sleuthing! I'd read someone say they were SiC, though couldn't remember if it was the SSII being referred to. (TBH I've always been slightly hazy on exactly what the difference(s) were, sounds like that might be one of them).
I’m tempted to fill it with Vaseline.
Haha... it might just work! The way they handle water is pretty unusual; they behave kinda like Crystolon stones, and they're often sold 'oil-filled' obviously.
Have you thought about sealing and permasoaking?