(FORTUNE Small Business) – One day about 20 years ago, when Janet Gurwitch was a young buyer with Foley's department store in Dallas, she got a phone call from a French guy, Georges Marciano. "I had never heard of him," says Gurwitch, sipping iced tea while browsing idly through her Cobb salad. We're at the new Smith & Wollensky steak house in Houston's Highland Village, around the corner from her office. Gurwitch, 50, has big eyes and long lashes and an accent that traces charmingly to her hometown of Hattiesburg, Miss. "He said, 'I have a nice jean. It's a tight fit, and I hear Texas will be a major market.' I said, 'We've got all the jeans we need--Calvin Klein, Gloria Vanderbilt. What's the name of these jeans?' He said, 'Guess.' I said, 'Sorry. It will never go anywhere.'"

You can guess the end of that story. Marciano persisted and finally won Gurwitch over, Foley's was soon selling more jeans than ever, and Gurwitch learned an entrepreneurial lesson that will never fade: Even in a mature industry, opportunities abound.

So ten years later, when Gurwitch told her fiancé that she was quitting her job as the No. 3 at Neiman Marcus (where she had succeeded Andrea Jung, now CEO of Avon) and launching a boutique cosmetics firm, and he asked, "Janet, do you think there's a shortage of mascara?" she didn't flinch. Today Gurwitch's former fiancé is her former husband, whereas Laura Mercier Cosmetics--the company Gurwitch co-founded eight years ago with Gary Kusin (now CEO of Kinko's)--has become a global brand projected to reach $90 million in sales this year.

"I was very comfortable in the corporate structure," Gurwitch insists. "I liked security. Yet I finally had an idea, and I decided to take my own money at age 42 and try something." From her front-row seat at Neiman Marcus, Gurwitch spotted a trend: Hot newcomer brands such as Bobbi Brown were elbowing aside old-guard fixtures such as Estée Lauder. On a business trip to London, Gurwitch watched young English girls and older Arab women crowd the MAC counter at Harvey Nichols. "It's not just Neiman's, it's not just Bobbi--it's happening everywhere," she says she concluded. "These new names are going to dominate."

Provided, that is, they can find a way to make consumers notice them. Guess succeeded with the help of a lavish magazine campaign featuring previously unknown German model Claudia Schiffer (photographed, oddly, from the waist up). Gurwitch didn't have the capital to fund that kind of debut. Instead she signed a licensing deal with Laura Mercier, a talented French makeup artist without much mainstream exposure, then leveraged Mercier's professional reputation in Hollywood (Madonna won't use anyone else) and Manhattan (for her work on fashion magazine cover shoots) to persuade the "woman of intelligence" that Mercier mascara was worth paying extra for. (Mercier's goods cost about three times more than Maybelline's.)

If I still sound as though I know what I'm talking about, you must be a man. I ask Janet to speak to me like the ignoramus that I am and tell me what the modern woman wants in makeup.

"I think she realizes that just because [supermodel] Amber Valetta did an ad, you're not necessarily going to look like that if you wear the same makeup. Blue eye shadow may not ever be good for certain people. The consumer wants what looks good on her."

"Do women wear as much makeup as they used to?"

"It varies by region. The South wears a lot of colored makeup. Florida, a lot. Texas, more than New York."

"You don't appear to be wearing much makeup at all."

"That's good--it works."