Optical medium
These materials generate, amplify or guide light, and they do it through electrons dropping between energy levels that a host material holds steady. What matters is the combination: the ion decides the wavelength and the host decides whether it can be made into a fibre, a rod or a crystal at all.
Erbium is the case that carries the modern world. Erbium ions in a silica fibre amplify light at exactly the wavelength at which silica is most transparent, which is what makes long-haul optical communication possible without converting the signal back to electricity along the way.
Material
Fused silica
transmitting from the deep ultraviolet through the visible into the infrared, which no ordinary glass does
Compound
Gallium nitride
as the emitting layer of every blue and white LED, and of the blue laser in a Blu-ray drive
Material
Indium tin oxide
passing around ninety per cent of visible light in the thin films actually used
Material
Optical fibre
the one that matters at scale: a signal can travel tens of kilometres in it before needing amplification
Material
Phosphor
converting one wavelength into another, which is what every fluorescent lamp, LED and X-ray screen does
Material
PMMA
transmitting about 92 per cent of visible light, which is more than ordinary window glass, and holding it over decades outdoors where polycarbonate yellows
Material
Polycarbonate
in spectacle lenses, where a high refractive index makes a thinner lens for a given prescription
Compound
Yttrium aluminium garnet
the standard host crystal for solid-state lasers, chosen for optical clarity, thermal conductivity and mechanical toughness together
Each of these uses is established and cited on its own. Whether something counts as an ore, for instance, depends on grade, price and era — and those qualifications are shown alongside it.