Decades after the De Beers Group started its “A Diamond Is Forever” campaign, the grandchildren and great-grandchildren of that slogan’s era are hoping to use those stones for their own engagements. And women in their 40s, 50s and 60s are looking for convenient and affordable ways to update their jewelry, or consolidate several stones into one piece. Gemvara, an online custom jewelry company, is exploiting a widening market gap. The mantra of reduce, reuse, recycle seems to be finding its way into the jewelry box. When Gemvara, which was founded in 2010, began offering stone resetting in January, demand for the service was overwhelming.