The best coil design to use in highly mineralized ground is a DD coil. Although any of the three frequencies will find gold, silver and copper, the best frequency for small gold jewelry is the 18.75 kHz. And it will do well on coins too.
By Beach Ground Balance I assume you meant you had it in Beach Tracking? If that is so, remember that Tracking or Beach Tracking is constantly updating itself, on the fly. You can check the "current" setting by pressing the GB pad. Just don't forget to press it again to go back to tracking mode. If you were simply using Auto Ground Balance, then switching to Beach Tracking, there is no reason to do so. Just put it into Beach Tracking to begin with and skip the Auto GB procedure. The reason being is because after you put it into a tracking mode and take a step or two, the ground phase is updated anyway.
The numbers you mention seem low for a DD coil. However, depending on what beach you were at, they are entirely possible. A smaller coil would probably ground balance with a higher number due to the amount of "mineralized sand" the detector has to neutralize at any one time.
It is difficult to get any consistency of GB readings in the air. Keep in mind that the coil is sending signals out, analyzing what comes back, and setting up what the software determines to be the optimum setting for that particular spot. That is how Auto GB works. If you do not have the coil above "the ground" when you hit Auto GB, it has nothing on which to base the signal reflection. As such, when ground balancing in the air, its any one's guess as to what type of reading you will get. And frankly, not accomplishing anything.
Next time you're at the beach, make sure you Noise Cancel first. Then turn down your Sensitivity to 15 or so, then put it in Beach Tracking. A lower Sensitivity might help in that highly mineralized sand. If you can operate without much chatter, boost the Sensitivity a couple clicks and see how it affects your ground phase reading. If you can still hunt without undue chatter, you can advance the Sensitivity a little at a time, until you obtain maximum performance. Let us know how it works out for you. HH Randy