Hester Diamond, a New York art collector, art dealer and interior designer who joined with her first husband in amassing an astonishing Modernist collection before tossing it aside in favor of old masters, died on January 23 2020 at her home in Manhattan. She was 91. Her son David Diamond said the cause was metastatic breast cancer. Ms. Diamond’s career spanned more than six decades, beginning with a part-time gallery job in the 1950s and culminating in the presidency of a research institute dedicated to Florence’s Medici family. She grew up in the Bronx and, as an English major, received a bachelor’s degree in 1949 from Hunter College in Manhattan. By then she had become a devoted museumgoer. The next year she married Harold Diamond, a Columbia University graduate from her old Bronx neighborhood. He was a fourth-grade schoolteacher in Harlem, and she took a job as a social worker. Eventually, Martha Jackson, an art dealer they had gotten to know, offered them weekend jobs at her gallery. Their sales were private — no publicity, no exhibitions — and, as one art publication suggested in 1970, they were handled with “the tact of a diplomat and the cunning of a spy.” Her specialty as an interrior designer was mixing antique furniture with contemporary art, and vice versa. In the 1980s her husband died at 56 after a very brief illness. She gave up the interior design business, having decided her vendors had become too unreliable. Her son Michael took the stage name Mike D, created the group the Beastie Boys with two friends and made hip-hop history. Ms. Diamond married Ralph Kaminsky, an economics professor. And she turned her collecting attention to old masters. After Mr. Kaminsky’s death, in 2012, she helped found Vistas (Virtual Images of Sculpture in Time and Space), an organization focused on new scholarship on European sculpture from the 13th through the 19th century, using both print publications and high-resolution online imaging. In addition to her sons David and Michael, she is survived by her third husband, David S. Wilson, a psychoanalyst, whom she married in 2015; a stepdaughter, Rachel Kaminsky; three stepsons, David, Daniel and Douglas Wilson; and four grandsons. Another son, Stephen Diamond, died of neuroendocrine cancer in 1999.