Agate has the same Mohs scale of hardness as Flint (black quartz), both have a hardness of #7.
Pirite (fools gold) will also work, it sparks well... It has a hardness of #6 on the Mohs scale.
I have also used a broken sharpening stone chip in my flintlocks, the hard stone is Agate or Chert, both will spark fine and does a good job, don't use any diamond stones though...
Anyway, Agate will ware the frizzen the same as Flint, so just treat it like flint and keep shooting.