Depends on what it was doped (not dyed) with. Dopants like magnesium will slowly move through the glass over time and with exposure to might or heat. These new

Depends on what it was doped (not dyed) with. Dopants like magnesium will slowly move through the glass over time and with exposure to might or heat. These new formations may or may not be optically active in our visible range. Some may be more or less intense or change color over time. Many metals may have many colors based on their state in glass, such as gold.