I'm curious about a stone I found on Ebay. It's not a one-time thing or an auction. A wood-working store is selling three types of natural Chinese finishing stone (three sizes that is) which seem rather cheap if the purported grit rating gives any indication of their performance. I'm curious if these are c12k's or something different as I've never seen them boxed/branded, and if so whether that might indicate more reliable performance/quality control.

Mujingfang Natural Water Sharpeningstone 13 000 Grit 66mm Wide | eBay

It mentions being cut from sedimentary rock in a quarry on the box