Lab created diamonds, whether Chatham created diamonds, Gemesis lab grown diamonds or any other gemstone for that matter, are the same actual material as their natural counter parts.
The only difference is the crystal is grown in a lab rather than in nature. The term for this is synthetic, meaning it’s the same material chemically, not to be confused with imitation. An imitation gemstone can be made of any material that ‘looks’ like the original. Imitations include cubic zirconia, glass, crystal and other inexpensive materials.