Sometimes natural stones are backed with foil or a metallic coating to enhance their color, to provide brilliance, or to produce a star effect. It is said that in an inventory of the Russian crown jewels by the Soviet Government, the ruby-colored Paul the First Diamond was discovered to be a pale pink diamond backed by red foil. Today, some diamonds are coated on the back with a blue film to improve their color. Aquamarine, when pale greenish blue, may be heated in order to deepen the blue color, and poorly colored amethyst may be heated to produce a beautiful yellow-brown quartz, called citrine, that often is misrepresented as topaz. By strong heating, the brown and reddish brown colors of zircon can be changed to blue or colorless, both of which states are unknown in natural zircon. Dyes, plastics, and oils are used to impregnate porous gems such as turquoise and variscite, and even jade. Off-color diamonds, when exposed to strong atomic radiation, can be changed to attractive green, brown, and yellow colors, causing them to resemble higher-priced _fancies_. In the constant search for something new, gem suppliers sometimes introduce into gemstones colors that are not always an improvement. For example, the beautiful purple of some amethyst can be converted, by heat treatment, to a peculiar green. Such an altered stone is marketed as _greened amethyst_. All of this tampering with gemstones complicates the problem of identification, so it is a matter of serious concern to the gem trade. SYNTHETIC STONES For over 200 years mineralogists have been devising techniques for producing synthetic minerals in the laboratory, and attempts have been made, sometimes with considerable success, to apply these techniques to the production of synthetic gemstones. To qualify as a synthetic gemstone the man-made product must be identical chemically and structurally with its natural counterpart. Sapphire, ruby, spinel, emerald, and rutile in gem quality have been brought to commercial production. Two of the basic techniques used in producing synthetic gems are the _flame-fusion_ and the _hydrothermal_ processes. [Illustration: The Verneuil furnace, for making synthetic gem rough. A mixture of hydrogen (H) and oxygen (O) burns almost explosively, heating the fusion chamber (F) to high temperatures. For example, powdered aluminum oxide and coloring agents are sifted down from hopper (A) to the fusion chamber and form a cylindrical boule (B) on an adjustable stand (C).]