I've been searching hard too.
I can confirm that quality clay does exist - my SE gets q44 clay with q80 ash but i did find one tiny patch on a clay area that gave q45. Useless but it proved they do exist.
It looks like the patches of q clay are tiny though - so just testing in a few spots is likely to mean missing them.
It is an incredibly frustrating part of the game - expecting us to spend hours and hours moving an SE a few feet each time to check for Quality clay..... We've given up trying to find any tbh. It's a game - not a job.
Same, i just create the highest qual furnace i can and just deal with qual 52 ceramic lol