Agree with bigjohn - - - just an opinion, but to me the word "guinea" is very tightly linked to (#1) the variety of the characteristic colors on the surface (the lavender, light green, blue, and on and on), whether they're sploched or striped or swirled or some combo of both, and (#2) the transparent base (of whatever color, clear, amber, cobalt).
I'd be willing to think of a marble as an opaque guinea if it had the same characteristic colors/distribution of colors as a "traditional" guinea . . . just on an opaque base.
To me the marble in question is a lovely two-color CAC striped opaque with a bit more action in the striping than usual. I'd think of it (and others like it) as a "fancy striped opaque" way before I'd go to the G word . . .
Even more, however, I wish it were mine! Nice, nice mib!
Sob.