Well it sounds like the novaculite is a chert, which is basically super-fine grained (microgranular) quartz. Some of the finer japanese stones sound like they are a mudstone. Mudstones can contain many mineral grains such as quartz particles and clays, but they have to be smaller than 62.5 microns. I guess for a whetstone the smaller and more uniform the grain size the better. You'd also want to find a mudstone that is not fissile or laminated (like shale)...it would have to be massive. I bet if you read formation descriptions on USGS maps it would give you areas to scout out.

P.S. I've seen a few barbers hones for cheap on ebay that are combination straight/safety hones. They seem to be smaller than the true straight hones (one I was looking at was 2.5"x2"). Would one of these be OK for refreshing a shave ready edge, or does the small size make it too impractical?

Cheers,

Xander