I finally beat it. I was able to clear the center channel down to one vine. Then I used a Blazing Speed to get most of the bottom row cleared. Then I used a star swapper and a detonator to finish it off.

Nope

Thanks! That worked for me.

I think I've figured it out. Star gems at the intersection of the imaginary lines connecting the gravel need to be exploded several times in order to see the gravel start to diminish. Also, if you explode two flame gems that are in a direct line with a gravel (and as close as possible to the gravel), a layer will be removed. However with only twenty plays, this is going to be tough.

Bejeweled stars level 421 - can't figure out how to remove the gravel.