Are lab-grown diamonds truly more environmentally friendly than their mined counterparts, as adherents claim? The Federal Trade Com­mission’s guides to eco-­marketing have always been clear: General benefit claims, like eco-friendly, should not be used. Yet, for years, lab-grown diamond companies have used that claim, and many others, to show the greater “ethics” of their product. And, with concern about climate change increasing, that positioning could prove potent. In March, the FTC put a stop to such claims. It sent warning letters to eight companies that sell lab-grown diamonds and diamond simulants, warning them not to call their products eco-friendly or sustainable.    “General en