If you aren't already doing so, a simple first step would be to brush the surface of the dough with oil before adding the sauce. The oil will serve as a barrier between the sauce and the crust and reduce the likelihood of the sauce migrating into the dough and creating a gum line (a doughy, dark-colored area just below the surface of the sauce). If you are using sugar in your dough, a second simple step is to reduce or eliminate the sugar. The sugar may cause the crust to brown quicker--before the rest of the pizza is baked--when baked on a hearth-like surface, like your Fibrament stone.
Another possible solution involves establishing a proper balance between the oven rack positioning of your stone, the oven bake temperature and duration, and the possible use of the broiler element. For example, in addition to implementing the "easy" steps mentioned above, you might want to place your stone in the middle rack position, lower the oven temperature by about 25-50 degrees F and lengthen the bake time (so that the crust bakes slowly and longer and crisps up better), and, if needed, move the pizza under the broiler for an additional minute or so of final top baking. (For a 6-7 minute bake time, I usually turn on the broiler element about 3 minutes or so into the total bake cycle.)
You might also find that using the stone on the lowest oven rack position (or possibly the middle rack position) and your normal oven temperature (500 degrees F), combined with the use of the broiler, may also solve your problem. Each oven behaves differently, so you may have to play around with yours to strike the right balance between the various possibilities mentioned above.
Another possibility is to dress your pizza on a pizza screen and put the screen directly on the stone. The metal of the screen keeps the pizza off of the stone and allows the pizza to bake without browning as fast. If you'd like, you can slide the pizza off of the screen onto the stone to allow for some bottom crust browning once the top looks like it has baked sufficiently. If you use this approach, you can also keep the sugar in the dough if you are using sugar. Again, you may have to play around with stone postioning and oven bake temperatures and times, as well as the possible use of the broiler element.
Good luck. Let us know if you solve the problem and what solution worked best for you.