Be safe is whichever method you choose. Face, eye and skin protection. Things can go sideways.
ETA: If you choose to grind the glass in any way - a respirator. Do NOT reheat the glass to a melting point.
It is a metallic sheen that was part of the glass. Attempts to remove it chemically or by buffing are unlikely to improve its appearance (I'm guessing that the base glass is their cheapest glass). But you could give it a go and see what you get.
Attend ONE marble show and they won't be mysteries any longer. You can learn in 2 days in person what will take you 1.5-2 years struggling to learn from flat, one-dimensional photos of varying lighting, focus and color balance on the Web.
They will need to be a person very well versed with contemporaries, as well as some good basis in vintage handmades and some minimal grasp of Jabos/DAS etc.
There aren't that many of those people about.
Note the wide black band edging. I think that is what you are referring to as it narrows.
An Akro employee told us that they used to throw bits of metal scrap into the pot just to watch it flare up. Amusement. I think pretty much of it all burns off.