The great value of gems led to their imitation in the earliest times. The Egyptians, who understood the art of coloring glass, made excellent imitations of the most costly precious stones known to them. The Romans used powdered rock crystal in imitating gems, and their counterfeits were so successfully made that it was difficult to distinguish them from the genuine article. The alchemists of the Middle Ages also produced excellent imitations of the emerald, ruby, sapphire and topaz. Modern imitations are of glass, usually known as paste or strass. This glass contains a large proportion of oxide of lead in its composition, and this makes it remarkably clear and brilliant. It is colored to imitate the most highly-prized precious stones, by the same process that is used in making colored glass (See GLASS, subhead Colored Glass). When clear, paste resembles the diamond, but it is soft and can be easily scratched, so that by testing it with a quartz crystal or piece of hard steel, it is easily detected. Imitations are also produced by using cheaper stones, having close resemblance to the genuine. Clear quartz and white Brazilian topaz, as well as colorless varieties of the sapphire and emerald, are often sold for diamonds. Cheap stones are also colored to resemble carnelians and agates. All of these imitations are skillfully prepared, and one not conversant with the methods of testing gems is liable to be deceived if he makes purchases of unreliable dealers. Diamonds and rubies have been successfully made by chemical process. The product was genuine, but the stones were all very small and the experiments were attended with so great expense as to make their production impracticable for commercial purposes.