There are two types of man-made substitutes, synthetics and simulants. A synthetic gem is made of exactly the same substance as the natural gem, but it is grown in a laboratory. It can sometimes be difficult to tell a natural gem from a synthetic one. Natural stones often have inclusions, which help with identification. Synthetics sometimes have lines or bubbles by which they can be identified. Occasionally, a substitute substance or gem may be passed off as the real thing. For example, a cheaper gem may be substituted for a more expensive one—citrine quartz can be substituted for the more precious topaz. Such substitutions are fraudulent. Simulants are not composed of exactly the same substance as real gems, but when one examines them, they appear to closely resemble the natural stone. Cubic zirconia (zirconinum oxide) highly resembles a diamond and may be sold as such. Imitation gems are often made of glass, plastic, or a combination of two (doublets) or three (triplets) substances. The parts may be imitation or real. Constellations and Gems Each constellation is ruled by a particular planet and is related to a particular gemstone: Twenty-seven Planets Gemstones Constellations Ashwini Ketu Cat’s Eye Bharani Venus Diamond Krittika Sun Ruby Rohini Moon Pearl Mrigashira Mars Coral Ardra Rahu Hessonite Punarvasu Jupiter Yellow Sapphire Pushya Saturn Blue Sapphire Ashlesha Mercury Emerald Magha Ketu Cat’s Eye Purva Phalguni Venus Diamond Uttar Phalguni Sun Ruby Hasta Moon Pearl Chitra Mars Coral Swati Rahu Hessonite Vishakha Jupiter Yellow Sapphire Anuradha Saturn Blue Sapphire Jyeshtha Mercury Emerald Mula Ketu Cat’s Eye Purva Shadha Venus Diamond Uttar Shadha Sun Ruby Shravana Moon Pearl Dhanistha Mars Coral Purva Badra Jupiter Yellow Sapphire Uttar Bhadra Saturn Blue Sapphire Revati Mercury Emerald
Gem Substitutes

(via Eastern Gemology)