Man-made diamonds are getting cheaper every year, which is to be expected of a product that can be produced at will.
image source: www.dcla.com.au
When a new man-made gem becomes commercially available its initial asking price is high. But, as soon as experts can separate it from the natural counterpart easily, its value drops significantly. In the early eighties, the well-known diamond simulant, Cubic Zirconia, confused many and were sold at about $20/carat. Today its sells for $1/carat.
A few years ago, lab-grown diamonds sold at 30% below the Rapaport Price List, which is universally used to value mined diamonds. Today, you can buy a GIA certified lab-grown stone for 90% below the prevailing Rapaport price.
The biggest problem with man-made diamonds is the heavy energy required to grow them – albeit on a smaller scale, a temperature as hot as the sun is required. Some scientists believe growing gems requires more energy than mining them.

The Gemesis diamond-growing factory in Florida
Diamond mining implies crushing rock and sifting through rubble, and does not require the use of dangerous chemicals, as in mining gold. And despite the disruption of nature caused by mining diamonds, a natural diamond is probably more eco-friendly than a man-made one.
Furthermore, lab-grown factories enrich a few entrepreneurs only, and offer no benefit to communities, such as jobs, schooling and health, as is the case with mined diamonds.

School boys at Renaissance Secondary School (image De Beers)
Lab-grown diamonds have no scarcity value and are difficult to re-sell.
Bottom line; man-made diamonds may become the cubic zirconia of the future. If improved technology can churn them out 24/7, they will become plentiful and near worthless. They are, however, the ideal gemstone to be used in fashion jewellery and should find a niche in the market for demi-fine jewellery.