“I thought it might be a piece of glass, it was so clear. I really didn’t know,” he said. Man Unearths 'Big, Ugly' 3.29-Carat Diamond at Ark. Park — the Latest of Over 400 Finds: 'Amazing ...

You have to add the rather mundane words, "in a vacuum." When light travels through matter - air, water, glass, or diamond, for example - it travels slower than 186,000 miles per second.