Yes, it's swarf, it's just embedded in the stone. Swarf by definition is just metal particles removed by a cutting or abrasive tool. The SG20k is not a friable hone - as I'm sure anyone who has used one will agree. The particles removed are very small, and get mostly locked into the surface of the stone. Some can be rinsed away with a little finger scrubbing still though. Those hones that don't embed particles much are usually friable stones, so that some particles of the stone come loose as they are dulled or packed up with swarf.