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Q. Why is there a difference of halachic opinion whether granite tabletops and countertops can be kashered?
A. It is well established that stone can be kashered (see Shulchan Aruch 451:8) and one would, therefore, imagine that all Rabbis would agree that granite can be kashered. However, granite is commonly sealed with a synthetic coating to prevent staining, and there is a difference of opinion as to whether that coating can be kashered. Some Rabbis follow the opinion that synthetic materials cannot be kashered and, therefore, rule that sealed-granite cannot be kashered. [A subset of this group is that some Rabbis follow this strict opinion for Pesach but not when kashering from non-kosher to kosher.] The cRc and most other hashgochos accept the lenient opinion that synthetics may be kashered; therefore, our Pesach Guide provides directions for how granite and other sealed stone surfaces can be kashered.
For more on the question of whether synthetics can be kashered, you may want to see Iggeros Moshe OC 2:92 & 3:58, Tzitz Eliezer 4:6:c, and Minchas Yitzchok 3:67.