Last weekend I made a bow drill bearing block out of a stone.
First of all you need to find a flat stone that fits nicely in your hand. The next thing you need to do is to start making a depression in it using some flint. 30 minutes later you’ll end up with a bearing block as shown above.
The advantage of a stone bearing block over a wooden bearing block is less friction. You want as much friction on your fire board, but a less friction as possible in your bearing block. Metal and glass are even better materials as a bearing block, but they don’t look as “natural” as a stone one.