Transparency comes down to electron energy gaps:

• Light is absorbed when its energy matches the gap between an electron's current level and a higher one
• In glass, that gap is larger than visible-light photons can bridge
• Visible light passes straight through because the electrons cannot absorb it
• Ultraviolet light carries more energy and does get absorbed, so ordinary glass blocks most UVB — which is why you rarely get a true sunburn through a window
• Metal ions shift the energy gaps: cobalt turns glass blue, gold nanoparticles make it ruby red, and iron makes it green