Thirty years ago, theoreticians predicted that ordered materials with a diamond-like structure would be able to control the flow of light in much the same way that semiconductors control the flow of electric current. These so-called photonic crystals could be used for optical switching and computing. Since then, researchers have made progress, devising methods for coaxing microscopic particles to line up and form various types of photonic crystals that function as optical waveguides and other devices. But due to synthesis difficulties, most of these materials have lacked the ideal cubic diamond structure and its coveted optical properties. A team led by Stefano Sacanna and David J. Pine of New York University has now come up with a colloidal chemistry method for making materials with the sought-after structure, advancing the field toward its goal (Nature 2020, DOI: 10.1038/s41586-020-2718-6). The researchers combined micrometer-sized polystyrene spheres with droplets of a polymerizable oil to form tetrahedral building blocks of four spheres surrounding and nearly hiding a single oil droplet. They used a solvent to deform the spheres, slightly extruding and exposing part of the droplet. Then they polymerized the oil and functionalized it with DNA, causing the other building blocks to adopt the orientation required for assembling in the diamond structure.