'Eye appeal' factors heavily into the value of a marble, but I think 'jaw drop distance' does in special cases. Right now my jaw is dropping at the marbles made by yet another talented contemporary glass artist. This time it is Mike Gong, who sells exclusively on eBay.
Pictured here is his 1.33" 'Persimmon Pulsator' which you must check out all the images to appreciate. Seriously, this marble could have fallen out of space from an alien planet.
He blends and complements his colors beautifully, crafting intricate, seemingly organic scenes. There are no harsh and unwanted contrasts here.
Mike doesn't have a lot up on eBay right now but he has a wonderful image gallery full of his works you can view here.