This photo shows marbling that would be sufficient to grade Prime.

The arrow points to the place where Fat Thickness (Fat or FT) would be measured.  This sort of fat generally ends up on the cutting room floor somewhere and is waste. 

FT is used to calculate the USDA Yield Grade along with the area measurement of the the muscle shown, the globular internal fat referred to as "kidney, pelvic and heart fat" (KPH) and the "Hot Carcass Weight" (HCW).

This photo shows marbling sufficient to grade "Choice."

Also note the fat thickness on this Rib Eye.  It is considerably less than the "Select" Rib Eye below.

This photo shows marbling sufficient for the "Select" grade.

Note the amount of fat thickness.  You would hope that a carcass that is this slightly marbled would at least be leaner.