Yes, you can heat treat agate slabs next to the fire if you keep the wind off the stone and make sure the heat doesn't go too much above 350F. If it doesn't heat well at 350F you can go higher next time. It doesn't take long and I usually try to maintain a good temperature, by steadily adding more coals, for about an hour and then let it cool down naturally.
I heat the stones by feeling the heat coming off the fire. I've got experience cooking at 350F - 400F on the grill so I can kind of judge the temperature. Otherwise, you'll need a thermometer or some other device to check the heat.
Don't let the coals touch the stones but it's ok if you can't help it. Just don't let big coals touch the stone. The stone should be just barely touching the ashes at the perimeter of the fire. Also, make sure you are only using coals for the heat. Flames are going to cook it too fast and when they go out the stone will cool too fast.
Try the technique on stone that's too small to work or contains defects before you put the good stuff next to the fire.
