Synthetic gemstones are made in laboratories or factories, not in rocks. They virtually have the same chemical composition and crystal structure as natural gemstones and consequently their optical and physical properties are very similar. However, they can usually be identified by the difference in the inclusions in them. Many types of gems have been synthesized but only a few are produced commercially, generally for scientific or industrial use.
Because of the way the synthetic gems are made, they show subtle differences in shape and colour that help to distinguish them from their natural counterparts. Similarly, the synthetic gems have inclusions, which are different from the natural inclusions. The experts can identify them and distinguish.

Synthetics are man-made gems often in the trade labeled as "man-made gemstone", "synthetic gemstone", "laboratory-grown gemstone", "created gemstone", "cultured gemstone" or, "reconstituted gemstone".

Synthetics can be exact copies of natural gems ie. synthetic ruby and synthetic emerald, which have virtually the same optical, chemical and physical properties as their natural counterparts.

Or those created materials which are exact copies of natural gems, but they are produced in colors not found in nature ie. blue quartz.

Or they can be unique materials which are not found in nature ie. cubic zirconia (CZ) and YAG (yttrium aluminum garnet). They are wholly manmade "artificial" gemstones with no natural counterparts at all, also known as homocreate.

An Imitation or a Simulant is any gem (natural or synthetic) posing as something else ie. natural white sapphire as diamond or cubic zirconia as diamond. Plastics and glass are often use as imitations.

Composite gems or Assembled gems are made of two or more pieces of gem material (mostly of not the same) joined together ie. doublet and triplet. They are joined for durable purpose or imitating (or faking) the top material gem.

Depending on the type of manufacturing process, synthetics have distinctive features.