But your geology teacher wasn't even correct. You can't make synthetic diamonds with similar color characteristics to natural diamonds at a fraction of the price. In fact, colorless synthetic diamonds are more expensive.
Not really true - it's relatively easy to make the diamonds colorless. Biggest difference is that the diamonds tend to have crystal structures that are a little rare in natural diamonds, since they grow very quickly compared to natural diamonds.
The price gap in natural vs cultured diamonds in the colorless range is far too narrow to make it a viable business model. So the effort, and profit, is made in the color end of the spectrum. Yellow and blue are the most popular, and the price gap is significant.