Applies to both
Natural and Lab-Grown
A lab-grown stone is the same material as a natural one. What differs is rarity, and what follows from it.
What it is
The same material, made differently
A natural stone formed in the earth. A lab-grown stone was made in controlled conditions from the same ingredients, with the same crystal structure, hardness and optical behavior.
It is not an imitation, and not a simulant like cubic zirconia — that is a different substance which merely resembles the look.
What it means for you
Price, rarity, and what you are buying for
Both are graded on the same scales by the same laboratories. A lab-grown stone typically costs substantially less for the same size and grade, which buys either a saving or a larger stone.
What you give up is rarity, and the resale that follows from it. Which matters depends on whether you are buying an object to wear or an asset to keep.
In practice
Telling them apart
That is a laboratory job, not an eye one. Reputable lab-grown stones are disclosed and usually laser-inscribed on the girdle.