Synthetic Diamonds such as Cubic Zirconia (CZ) and moissanite are made of zirconium oxide and Silicon Carbide respectively. But moissanite and cubic zirconia are chemically different than Diamonds (Earth-mined and Lab-grown diamonds), and don't quite look the same as a result; cubic zirconia has less brilliance, or sparkle, then diamonds and moissanite produces a blurred effect called a double refraction.
Each MPCVD reactors cost more than half a million dollars and needs continues Research & Development Expenditure which also increases investment.
It is possible that the reason that jewelers promote this misinformation is that some cheaply produced cubic zirconia will oxidize and get cloudy over time. Cubic zirconia (a.k.a American Diamond) is not a diamond.
Moreover, the machinery required to duplicate the conditions 100 miles below the Earth’s surface is very expensive, each press costing between USD 500,000 and USD 1,000,000 each plus infrastructure.
Mined diamonds are “free”…you can just pick them off the ground in some areas but all the “added value” comes from marketing expenses added on. On average, natural diamond costs USD 200 per carat to mine from the earth! Not so Lab-grown. We each have our own up-front costs.