Alexandrite is a type of gem used mainly in jewellery. It is a form of chrysoberyl and comes mainly in green, yellow or brown. It was discovered in the early 19th century in the Ural Mountains in Russia and was named after Alexander II. It is very unusual and is extremely valuable because of its rarity. Natural alexandrite changes colours when viewed in different lights. Many companies have created synthetic alexandrite that is typically made from synthetic corundum or spinel. Certain versions of created alexandrite gemstones are easy to spot but because of technology, most manmade versions are very hard to detect and require a lot of knowledge and expertise in the gemstone industry.
Know the difference of the properties. Natural alexandrite has certain chemical and physical properties. Created gemstones are generally made two different ways. Simulants are manmade gemstones that do not have the same chemical and physical properties of the gemstone they are trying to replicate. Synthetic gemstones on the other hand are replicas that have the same appearance, chemical formula and crystallographic structure.
Look at the size of the gemstone. Large clean alexandrite gemstones are extremely rare and expensive. Most likely, if the gemstone is really large and clean, it is probably an imposter. Large natural alexandrite gems are not impossible to find, but because they are very rare, most large gems end up being fakes.
Study the colour of the gem. Natural alexandrite gems are yellow, green or brown. Almost all of the natural ones contain the colour green. A lot of synthetic alexandrite gems contain synthetic corundum, which is used to produce colour changes within the gem. This synthetic material has a purple or mauve like colour to it. Because this is used in many synthetic alexandrite gems, there is no green. Without the green colour, it is most likely a manmade gem.
Examine the gem closely for other factors. Many synthetic alexandrite gems contain gas bubbles, flux inclusions or curved striations. These are common problems that occur through the production of synthetic gems.
Have it tested. If you are still unsure of the gem, have it tested by a gemologist. A gemological test will indicate if it contains chrysoberyl, making it a real alexandrite, or if it has another material in it, such as corundum.