Materials inspired by nature comprise a running theme of modern science. Among the crystals that can be formed, diamond is perhaps most emblematic. In the conventional thinking, natural diamonds form only under high-pressure and high-temperature conditions. Here we show a new, natural form of diamond crystals of high quality that are epitaxial with their ruby substrate. Diamonds in rubies are rare; heteroepitaxial diamonds are twice as unexpected. Epitaxy suggests that the natural diamonds in the rubies were formed after ruby crystallization in a thermodynamically diamond stable region. This striking natural control over diamond epitaxy suggests a general strategy by which to form naturally-inspired, gem-quality crystals.