Turned a couple of these blanks a few years back with my carbide tools. Yes, I did notice mostly just "powdery" material as opposed to ribbons coming off. I dry sanded with my usual sequence up to 600 grit and wet sanded with MM thru 12,000, then polished with Novus 2.
What I noticed was a difference in the glow intensity between the upper and lower barrels. I believe they were Jr. Gent II pens and I suspect the "glow" material was not evenly distributed throughout the blank material. This also happened on a couple of PSI's M3 Ti Mokume pen rods I turned around the same time. So, my recommendation is to use a pen kit where the upper and lower barrels are the same diameter (or as close as possible). Or, use a single tubed pen kit. Maybe others have noticed this uneven distribution of material within the blank or hopefully there have been improvements in casting during the past few years.
Jeff