[LB061303], Letter from Thomas Alva Edison to Edward Goodrich Acheson, November 19th, 1894
https://edisondigital.rutgers.edu/document/LB061303
Transcription
Nov 19 1895 E G Acheson Esq Your letter received – in reply would state that to any inquiry from a party in Boston I did state that we had tried Carborundum at the phonograph works and found it unsuitable for cutting Saphire We pick out the hardest Ceylon Saphire & grind with diamond dust, This kind of Saphire [will?] abrade Corundum like chalk & I do not believe anything on earth but diamond dust will cut it Yours Thos A Edison