Hey, I get rings like this even with sanding, but it's from using old nearly worn-out sandpaper! The heat buildup can cause a change in the wood coloration. This is most evident on woods that are sensitive to such changes, like purpleheart, maple, ect ... Maple turns a darkish brown to black ... purpleheart, unless you heat treat it yourself later, will have a spectacular purple shimmer deep inside the wood at the ends but will otherwise look black, especially in dim light indoors. I prefer to take a small blowtorch and heat treat my entire purpleheart blanks to get this appearance all the way through. It looks nearly black indoors, but when they take it outside it REALLY turns heads.
Almost forgot ... how do you fix it???
If using a pen mill, make sure it's properly sharp, and that you're running it in forward gear. (I know, that sounds kinda stupid, but hey, it can happen! DAMHIKT)
When working on the lathe, make sure you're still rubbing the bevel with your lathe tools as you work, but remember that it really only takes LIGHT pressure to make your cuts. You don't want to burnish your wood with the bevel of your tool. Knocking the speed down a bit can help, but the best help you can do is to make sure your tools are razor sharp and properly maintained, and to polish the backs of your lathe tools so that the bevel creates as little heat from friction as possible.
Use fresh sandpaper for your sanding needs ... and a light touch rather than bearing down will yield best results... older paper will leave scorch marks, or even metallic or off-color wood residue behind in your work. Sandpaper isn't THAT expensive ... use fresh paper with every project unless you don't care about the final appearance. Also keep in mind when inlaying metals and stone in wood, you want CA EVERYWHERE ELSE so that the dust doesn't get into the wood while you work.
Micromesh is somewhat of an exception in this case ... you're only going to use it on acrylic or on a CA based finish, right? So it should NEVER touch anything but plastic... no issues there. You're also wet-sanding with it, so the residues are washed away.