" Nothing worse than gauging a new stone." Yea - dropping a new one on a tile floor right out of the box is worse
Pretty accurate advice already given. The SS10k is the softest of all. The 12k is harder. The Chocera 10k is a good bit harder than the 10k SS and has a much higher grit density.The Shapton 15k Pro is quite hard. IMO the 16k Glass is a bit harder. The 30k Pro is very hard. The newcomer on the block is the 15k Nubatama Bamboo. Harder than the 10k or 12k SS and leaves a finer finish in the range of the Shapton 15k Pro and 16k GlassStone, perhaps a bit finer (hard to say). It develops a fine mud unlike the Shaptons ( more like the Naniwa stones), yet it imparts a high contrast finish unlike all the other stones mentioned here. A very unique stone. Certainly the stone of choice for single bevel carbon steel knives, only surpassed by some natural stones in finish. Expensive compared to all but the Chocera 10k, but it is in a separate class. Totally addicting. Check out the video on Mark's site.