Does this line in my sapphire mean heat treatment?

[bigsky_wanderer]

Does this darker colored line suggest this sapphire has been heat treated (or treated by some other non-natural method)? Or do lines like this form naturally in corundum? Aside from this single, thin, darker colored line, the sapphire seems to have consistent coloration throughout. This stone had passed through several sets of hands before it came to me, so the extent of treatment is unknown. Thanks for any insights!

 

[Gloria27]

Nope, those lines are natural growth lines within the sapphire crystal.
Heat treatment is detected by studying the inclusions inside the sapphire, because gems are not perfect and sometimes other type of crystals grow inside them. When heat treated, these crystals expand and around them is formed what is called a decripitation halo. This is the tell-tale sign of heat treatment. In other instances, other inclusions like rutile needles can tell if a sapphire was heated or not.
Someone please correct me if I'm wrong.

 

[bigsky_wanderer]

Thanks for the response! Whoever cut the stone did a nice job tucking this line toward the outside, under some of the facets. This growth line is not obvious unless you scrutinize the stone from beneath.