Sunlight is often better. Make sure you are manually selecting focus on the marble itself - which also determines exposure.
Artificial lights have their own varying color temperatures. Those temperatures add artificial color to the photo, or fail to bring out true colors.
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.