Short answers, pulled from the story.
Pliny described the use of several naturally occurring stones including oil and water stones used by craftsmen across the Roman empire. He recorded locations of ancient sources for these materials which were utilized throughout the empire.
The Ouachita Mountains in Arkansas became a noted source for novaculite stones known as Arkansas stones. Novaculite crystals measure between 3 and 5 microns making them suitable for fine stages of knife sharpening.
The legendary Honyama mines in Kyoto Japan closed their operations since 1967 due to this shift. The widespread availability of high-quality artificial stones has diminished the use of natural stone for sharpening.
The Japanese traditionally use natural sharpening stones wetted with water rather than oil. Using oil on such a stone reduces its effectiveness significantly compared to water usage.
Shiage-to reaches 7000 to 10000 grit while naka-to likely falls within 3000 to 5000 grit range. Ara-to probably measures between 500 and 1000 grit using non-Japanese grading systems.