Taken from “On the Main Line“:
The story which, again, is contained in the Landau biography, goes something like this – but before I give it, here is what it is preceded by, which is worth quoting for it implicitly acknowledges the absurdity of the story, even as it somehow gives it credit for being a story of “the time,” presumably the time of the Gaon himself:
The following story gives one a sense of how people of the time understood the conflict between the Gaon and the maskilim. It also illustrates the naive view people had of science-a weakness the Gaon tried to ameliorate.
The story is that the curators of a Russian museum had obtained some kind of precious mineral which reputedly possessed remarkable powers. Scientist could not figure out the secret of the stone, so they decided to send it to Germany, where the real great scientists were! They also couldn’t figure it out, among them was Moses Mendelssohn, who suggested that they send it to the Vilna Gaon. Two German professors (Shulman says they were short and bald!) went to Vilna and brought the stone to the Gaon. The latter instructed his shammash to bring him a glass of water. The Gaon dropped the stone into the water and – poof! – the water disappeared. Seeing the shock, the Gaon explained to the German scientists that the stone was a sapphire and that when water comes into contact with sapphire the two elements of which is is composed separates and it reverts back to hydrogen and oxygen.
When the dumbfounded scientists returned to Germany and told everyone about the amazing rabbi in Vilna, Mendelssohn was excited and exclaimed that with this we can understand a naturalistic explanation for the splitting of the Red Sea! Moses’ staff was made of sapphire, according to rabbinic tradition. When Moses split the sea with his staff, it was through the alchemic properties of sapphire!
When the Gaon was told of this “he was outraged. “Heretic!” he proclaimed.” He then explained that the verse says harem et matekhah lift your staff and neteh et yadekha al hayam stretch out your hand over the sea (Ex. 14:16). Harem here means lift, but it also means to remove, set aside. So what actually happened was that God told Moses to set aside his rod and then stretch his hand over the sea – specifically so that the miracle was supernatural. This indeed was the miracle! He had a sapphire rod, but he used his hand.
Shulman often gives sources and here too he writes that it is to be found in Hagaon Hachassid pg. 375. The story is completely missing from the Rosenblum book published by Artscroll. You can see that Shulman himself knows it’s bunk, hence the preface “the naive view people had of science.” He doesn’t spell it out further, doesn’t note how even aside from the oddity of the belief that sapphire can separate water into hydrogen and oxygen, that the chemical composition of water was not even known until (shortly) after the Gaon died (link). Unless, of course, the Gaon already had discovered it. Interestingly, in the preface to the book Shulman relates all the sources he used, the ones you would expect. However, he pointedly writes that one famous work, “Toldos Hagra was not used despite its many anecdotes, because no sources are given.” The whole thing is remarkable. And there are sources for this? As I said, Shulman himself has to obliquely note that the story is obviously a tall tale.