Lapping Problems

Today I was using a DMT 220/325 plate to lap a Japanese 6000 grit stone that I have had for a while, but never used. After about ten minutes, I started to notice rough patches on the stone. Now the entire surface of the stone is covered in pits, and bits of stone came off when I rubbed it with my finger. Is the stone faulty, or did I do something wrong? Should I just keep going and hope that it gets better farther down? I have never had any problems lapping my Nortons, but I am new to all this and could use some advice.

Is it a ceramic stone like the Nanaiwa & Shaptons. They are a more sensitive to external stimuli, ie excess heat, cold, water, detergents & so on.

No, it is not ceramic, I think it is just an artificial water stone.

Then you may have a warranty claim if it is new & you cannot lap past these areas easily. Waterstones should not crumble from lapping.