Learn Turkish: Benim kolyem ağır ama seninki hafif. - My necklace is heavy, but yours is light.

It is built by combining sen (you) + -in (the genitive suffix, meaning 'your') + -ki (the possessive pronoun suffix). Added together, seninki means 'yours'. It acts as a pronoun so we do not have to repeat the word for necklace.

Could I just say senin kolyen instead of seninki?

Yes, you could say ama senin kolyen hafif. However, just like in English where we prefer 'yours' over repeating 'your necklace', Turkish uses the -ki suffix to sound more natural and avoid repetition.

Could I use -ki in the first half and say Benimki ağır ama seninki hafif?

Grammatically yes, that means 'Mine is heavy, but yours is light.' However, you usually need to establish what you are talking about first! By saying Benim kolyem (My necklace) in the first clause, the listener knows exactly what the -ki in seninki is referring to.

Why is it kolyem and not just kolye after benim?

In Turkish, when you use a possessive word like benim (my), the noun it owns must also take a possessive suffix. Since kolye ends in a vowel, it simply takes the first-person singular suffix -m to become kolyem.