It’s hard not to be mesmerized by the marble island that takes center stage in Australian designer Tamsin Johnson’s home in Sydney. Carved from a striking hunk of gray Bianco Gioia stone, the audacious design features a gravity-defying counter perched on plinths of marble stacked in a jagged-edge arrangement. The result may look like minimalist sculpture, but it’s not just for show. The island is fully kitted out with everything from drawers to power outlets. “I love spaces that are equal parts functional and beautiful,” Johnson says. If her home has a fashionable feel, it’s by design. Both she and her husband, Patrick Johnson, have roots in fashion. Tamsin studied…