There is a difference between simlated and synthetic gemstones. Of course a natural gemstone is a genuine gemstone.
Simulated gemstones are manmade gemstones, that can be made of any materials, including glass. They mimic the natural gemstone, they are simulating. They are often very large, affordable, clean and quite beautiful. They do not hold a lot of value, but make quite striking jewelry. Probably the most well know simulated gemstone is a CZ, which of course is a simulant for a diamond. Another one that I very much like, is the Paraiba Ice. It is a simulated gemstone made to mimic the very expensive Paraiba Tourmaline, which is the natural form of the gemstone. It has a lovely swimming pool blue color, and is quite affordable, in the simulated form.
Synthetic gemstones, are manmade gemstones as well. However they have the same chemical and optical properties, as a natural gemstone. There are some that carry a great deal of value, and are worth having in a collection. I own some padparadscha sapphire synthetic gemstones. I would of course love to own the natural form, but it is not only extremely rare, but extremely expensive. There are many synthetic stones on the market. It is like having a perfect version of the natural stone, without the expense.
Of course, as a collector, I would love to have a natural gemstone in all the stones that are available. Unfortunately that is not always possible, so these lovely substitutes, do nicely in their place.
