According to Wikipedia:
Reading stones were said to be invented by Abbas Ibn Firnas, in 8th century Córdoba, Spain, who had devised a way to finish sand into glass; which until this time, was secret to the Egyptians. These glasses could be shaped and polished into stones used for viewing - known as reading stones. These spread to the rest of Europe from approximately the 11th century onwards. The function of reading stones was replaced by the use of spectacles from the late 13th century onwards.