With a higher relative hardness, man-made aluminum oxide stones are the modern Japanese water stones and India stones the difference is in the bonding. The material was originally found in nature "but the manufactured abrasives have dominated since the early 1900s," according to "Sharpening Made Easy" by Steve Bortorff of the Ohio Knifemakers Association.
Stones made of silicon carbide include some ceramic-bonded stones and the Norton Crystolon stone pictured. They'll sharpen anything except carbide-tipped tools, writes Bortoff. Try gluing a piece of silicon carbide wet-or-dry sandpaper to a wood block for an inexpensive sharpener.
Natural Arkansas stones work well on blades with high carbon content. The original stones, also known as Ouachita (or Washita), were milled, but now many labeled natural may be of a "reconstructed" material called novaculite, Bortorff writes. Novaculite is related to flint and quartz and contains primarily silicon dioxide. These stones work well on carbon steel knives but "struggle with harder tool steels and tougher wear-resistant and stainless steels," Bortoff writes.
Industrial diamonds are bonded to steel to form sharpeners with the highest relative hardness. The higher-priced monocrystalline diamond hones last longer because the diamonds don't fracture as readily, according to Bortoff. Mad Dog Knives owner Kevin McClung uses diamond hones because they don't need oil and require little maintenance.