This can be 1 of many things that would cause a "dimple"
* It could indeed be a chip in the slate - Have a professional repair the chip by using wax or plaster.
* There may be a indention in the cloth itself - This is normal if you masse alot or jump balls on your table.
* Some tables have a center slate screw which prevents the slate from seperateing from the frame. There is a possibility the installer used wax/plaster to fill the hole and a ball or agressive shot left an indention in the filler.
Thanks for the reply. After re-reading my post I meant to say the indention was under the foot spot, where you rack the head ball. Does that paticular spot have any predisposition for chips or cracks ?
People will commonly tap the head ball to ensure a tight rack. This could possibly cause a chip but unlikely. There is a good chance the cloth or the spot has an indention in it.