All quartzites are very hard. They are even hard on the diamond abrasives - just like Arkies. If you're using a diamond cut off wheel at all you should use oil lubricant to cut them - otherwise the wheel will glaze over pretty quick. This has to do with the metal bond holding the diamond getting very hot and plastically flowing over the diamond, preventing it from cutting properly. Using a plain diamond lapping plate you are still good with water though. And a lot of elbow grease.
I recommend loose grit lapping with SiC abrasive, much faster.