While many sapphires may exhibit faint color changes upon exposure to incandescent or fluorescent lights occasional stones may display dramatic changes. Most color change sapphires belong to one of two groups depending on their color change; the green to red (alex type), and the blue to purple kind. Most alex type color change sapphires are from Songea, Tanzania while the blue-purple types occur in Madagascar, Sri Lanka, Burma, and other parts of East Africa and Tanzania.
In all color change stones, the strength of the change is probabably the most important factor affecting the value of the stones.
Some people prefer the green to red change while others prefer the blue to violet change. The blue to violet change is probably worth more but there are exceptions and a stone with a strong green to red change would also be quite valuable.