Often the average consumer interchangeably uses the words man-made/simulated and synthetic gems thinking that it means the same, however, there are significant differences between the two.
The Egyptians were the first to produce man-made gems from glass. While the term man-made is commonly used, the term simulated gems means the same thing. These laboratory created gems look like the actual stone, especially in color, but do not have the characteristics of a genuine gemstone, i.e. crystal structure, stone hardness, natural inclusions. These stones are usually made from plastic or glass. A good example would be cubic zirconia which is a good impostor for genuine diamonds.
Synthetic gemstones, while they imitate the color of the actual gemstone, also have the similar characteristics of the gems. These characteristics include having the exact same crystal structure, transparency, natural inclusions, optical and chemical properties.
Until the proper equipment was developed to test the genuine from the fake, it was difficult even for seasoned gemologists and jewelers to differentiate. Today, there are sophisticated tools to distinguish the real from the faux.
As an example, we have a tool that we use to distinguish a cubic zirconia from a diamond. When it emits a small beep upon touching the surface of a diamond, we know that it is definitely 'a girl's best friend'. When a skeptical young lady comes in to have her diamond tested and the tester does not emit a sound, my message is, "boyfriend, you better start running for the hills'.