In the jewelry world, like art, fashion and other markets, fakes are often passed off as the real thing.
Some are obvious, some not so much. In the gem world, there are three kinds of stones sold as diamonds:

I - Naturally occurring diamonds. These are diamonds found in diamond mines which are the product of natural processes of an immense amount of time.

II - Fake diamonds. These are stones which superficially look like diamonds, but are really some other kind of substance. Stones made of glass, paste and quartz crystals have been substituted for diamonds, particularly when a close examination is not likely. The term costume jewelry comes from the practice of using these phonies in theatrical costumes to create the effect of wealth without the expense.
Some of these fakes, like zirconium, are gem stones in their own right, but are not made of the same materials and don't have the same crystalline structure of a true diamond.

III - Artificial diamonds: These diamonds are not, strictly
speaking, true phonies. Artificial diamonds are man made stones produced by duplicating the process whereby natural diamonds are made.
A diamond is a crystal formed from carbon under extremes of pressure and heat.
Artificial diamonds are formed much the same way, except instead of being the result of tectonic forces, they are grown in a laboratory.

If artificial diamonds are chemically and structurally the same as natural diamonds, can they be considered real?
Yes, in one sense, they are as real as any diamond from a diamond mine.
But in the gem world, rarity is a commodity that is part of the total value of a diamond. There are only so many naturally occurring diamonds in the world. This rarity adds status and value to the natural diamond that artificial diamonds will always lack.

As long as appearance is the primary factor in choosing a diamond, the man made article will do just fine. Often, in fact, they will be of a higher quality then their natural cousins, as they did not have to contend with the vagaries of competing natural forces in their creation.