A “bearing block” is any device that is used to bear down on the top end of a spindle to facilitate the stability and pressure of the spindle as it is spun using a bow or strap. There is usually an indentation that keeps the tip of the spindle in place. The best feature of a bearing block is that it is hard and durable while at the same time being as friction free as possible.
As illustrated in the photo some rocks have a natural indentation that will stabilize the rotation of the spindle while in use. Rocks of this nature are often found by lakes, streams, and seashore. Other rocks may have angular crevasses that can be used as well. The material of the rock may vary in surface from smooth like agates to gritty like sandstone but most often has the advantage of not transferring heat to your hand.
For more information regarding bearing blocks in general check out my post “Bow And Spindle Bearing Blocks”