Lots of good info with simple Google search..try; COLOR OF FLINT ROCK.
Color is imparted by the nature of the surrounding sediments. Black to dark is organic coloring, red > brown > yellow, variations of iron oxide.
"clarity" is basically a function of the fineness and density of the crystaline structure. So those magnificent Nueces valley translucents are a high grade flint with just a touch of iron oxide.
Related info;; In pure geology speak, all flint is Chert...but not all Chert is flint . . . . . . Real chert is generally course and dull. Flint is shiney.
Aside from sponges, another large flint variety is DIATOMS...very small micro creatures that were swept into huge flat deposits by wave action.
For those few that are really interested in this subject....there is exactly just such a structure right at the C3 Camp 13...
When Richard dug the "test trench" that is now a major producing area, he dug far up hill....No body bothers with this area now as its poor habitation material but...at about the 2 foot down level is this sheet flint. Just look at the pieces in the throw dirt.
Flint ridge Ohio is very special....said to be 98.93 silicon dioxide. ( very high level of pure creature )
The word is out
There are those that study / collect varieties of flint. As amber is reknown in the Baltic area so the banded flint of Poland is unique. Also high on the honerable mention list is Flint Ridge Ohio and Alibates quarry TX.
Complete identifiable inclusions in flint is very common, the banding itself is not the design of the creature...
Think of the best banded flint once being the consistency of toothpaste, slightly different densities will gel harder than the rest, and Like paint, outer areas will skim over first then subsequent movement will blend this into the main body...
The holy grail of flintery might be a complete Mossasaur in flint