Real Gemstones which can be had from a Gemstone specialist can influence a lot of things in our lives. They are not really worn for protection but they are worn according to date of birth, name etc. They are worn for many purposes; health reasons, luck reasons, personality reasons etc.
The real stone may change its colour because of many reasons but mainly if a person is going in a problem or about to go in a problem. A real gemstone will never change its colour due to dirt/dust, but the ring on which its embedded can get dirty or dark.
Hazrat Muhammad (saw) used to wear cornellian (akeek), and thats also a reason why many people wear akeek regardless of their date of birth but since 90% buy fake ones through fraud by seller. the stone has no effect on them.