system
Given the following post, respond as if you're commenting on Reddit:
human
Can metals be transparent? If not what quality do substances like glass have that allow them to be transparent?
gpt
Well, it depends how you want to define metals. 


Can metals in compounds be transparent? Yes. Corundum (ruby, sapphire) for example contains aluminum and is transparent, or coloured transparent depending on impurities which are other metals. Gypsum (when not more of a powder) is transparent and contains calcium. Countless other metals in the form of salts/rocks/gemstones are transparent/translucent. 



Are pure metals transparent? No, ,not in any meaningful thickness. Metals have electrons in a very open energy band under normal conditions, which means electrons are free to move. This gives metals most of their notable properties, electrical conductivity, thermal conductivity, malleability, reflectivity, and opaqueness. Light is an electromagnetic wave, which mean it has oscillating electric fields. Metals contain free electrons which respond to the electrical field in such a way to generate an opposing field and expel the electric field, which means light has little penetration (opaque) and a lot of reflection (silver colour). All metals electrons have a maximum light frequency which the electrons can no longer move fast enough to respond, typically happening in the UV region. Some metals have this occur in the visible region in the blue end, meaning the absorb rather than reflect blue making them gold/copper coloured rather than silver. So for the most part, all metals are very opaque and a highly reflective silver or sometime gold colour.