The sale price of faceted gem synthetic sapphires varies greatly, just as crystal growing processes do. These items can cost anywhere from $1 to $1,000 each piece. Sapphire's price is very volatile because it is more sensitive to changes in its market demand than other precious stones. Synthetic sapphires are manufactured by heating aluminum oxide at 2,700 degrees Fahrenheit (1,443 degrees Celsius) under high pressure. The heat turns the aluminum oxide into sapphire, which is then cooled with water. The finished product is clear and colorless, although some colored varieties exist. It may be blue, green, red, or yellow. Sapphires have been used for jewelry making since at least 500 B.C., when Greek historian Herodotus wrote about them. However, it was not until much later that they became popular as jewelry materials. In the 17th century, Portuguese traders sold large quantities of sapphires off the coast of India. When they returned a few years later, many of the traders were unable to pay for their purchases so they traded salt for food instead. This may have helped sapphires reach Europe, where it was admired for its beauty. Since then, sapphires have become one of the most popular jewelries materials out there.