Due to their iridescent colors, opals have been considered particularly precious gemstones since antiquity. The way these stones shimmer is caused by their nanostructures. A research group led by Prof. Dr. Markus Retsch at the University of Bayreuth has produced colloidal crystals mimicking such structures, which are suitable for constructing new types of sensors. These sensors visibly and continuously document the temperature in their environment during a defined period. They are, therefore, tailor-made for a permanent monitoring of temperature-sensitive processes. The scientists have presented their discovery in the journal Advanced Materials.