The article focuses on the complex interaction between gold and light in the very specific medium of mosaic in Late Antiquity and Byzantium. Studying the metaphoric qualities of light/gold and its qualitative distinctions in early Christian and Byzantine sources leads us to an understanding of the complex function of gold or golden tesserae in script and images. As gold is understood as light, mosaic seems to be a more or less perfect medium as it is not stable, but dependent on the changing light. The gold ground is transformed in every moment by light. Mosaic can thus be understood as the medium with the greatest capacity to bridge – virtually – the gap between the visible and the invisible divine light.