This is a typical item. It's called the Boncuk, the Little Magic Stone that protects from the "Evil Eye" (pronounced "bon-dschuk"). This blue glass piece is everywhere here in Turkey.
But what is behind this superstition?

I will try to explain in a shortened version. Once upon a time (yes, it starts like in a fairy tale) there was a rock by the sea that, even with the force of a hundred men and a lot of dynamite, couldn't be moved or cracked. There was also a man in this town, who was known to carry the evil eye (Nazar). After much effort and endeavor, the town's people brought this man to the rock, and the man, upon looking at the rock said, "My! What a big rock this is." The instant he said this, there was a roar and crack and instantly the immense and unbreakable rock was found to be cracked in two.

The force of the evil eye (or Nazar) is a widely accepted and feared random element in Turkish daily life. The word "Nazar" denotes seeing or looking and is often used in literally translated phrases such as "Nazar touched her," in reference to a young woman, for instance, who mysteriously goes blind. 

Another typical scenario. A woman gives birth to a healthy child with pink cheeks, all the neighbors come and see the baby. They shower the baby with compliments, commentating especially on how healthy the baby is. After getting so much attention, few weeks later the baby is found dead in his crib. No explanation can be found for the death. It is ascribed to Nazar. Compliments made to a specific body part can result in Nazar. That's why nearly every Turkish mother fixes with a safety pin a small Boncuk on the child's clothes.
Once a Boncuk is found cracked, it means it has done his job and immediately a new one has to replace it.

Source