The Nimrud lens or Layard lens is a 3000-year old piece of rock crystal, which was unearthed by Austen Henry Layard at the Assyrian palace of Nimrud, in modern-day Iraq. It may have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight, or it may have been a piece of decorative inlay. The lens is slightly oval, and was roughly ground, perhaps on a lapidary wheel. It has a focal point about 11 centimetres (4.5 in) from the flat side, and a focal length of about 12 cm. This would make it equivalent to a 3× magnifying glass...More at Wikipedia.
The function of the lens is not clear, with some authors suggesting that it was used as an optical lens and others suggesting a decorative function. Assyrian craftsmen made intricate engravings, and could have used a magnifying lens in their work. The discoverer of the lens noted that he had found very small inscriptions on Assyrian artefacts which he suspected had been achieved with the aid of a lens. Italian scientist Giovanni Pettinato of the University of Rome has proposed that the lens was used by the ancient Assyrians as part of a telescope, and that this explains their knowledge of astronomy. Experts on Assyrian archaeology are unconvinced, doubting that the optical quality of the lens is sufficient to be of much use.