Marcel Tolkowsky was a Belgian diamond cutter and gemologist, best known for developing the modern round brilliant cut for diamonds in 1919. His innovative design maximized the stone's brilliance and fire, making it a popular choice for engagement rings and other fine jewelry. Tolkowsky's work is detailed in his book, "Diamond Design," where he outlined the mathematical principles behind his cutting techniques. His contributions significantly influenced the diamond industry, and his cutting style remains a standard in the trade today, impacting how diamonds are evaluated and appreciated worldwide.