A lab-grown diamond gets talked about as if it's a diamond-shaped substitute, something else made to look the part. It isn't a substitute. It's grown in a lab, not dug out of the ground, using one of two processes that recreate the exact conditions that form a diamond underground.
High pressure, high temperature (HPHT) compresses carbon under the same pressure and heat found deep in the earth. Chemical vapour deposition (CVD) builds a diamond differently: layer by layer, from carbon-rich gas in a chamber.
Either process compresses into a matter of weeks what would otherwise take roughly a billion years underground.