Question: Allah does not forgive "shirk" - right? What is "shirk"?
From the Collection of Montheism (Islamic "Tawheed")  (Part 5)

"Is it considered to be "shirk" when someone wears amulets, rings or strings, etc. to remove afflictions or to prevent evil things from happening? "

Answer: "Allah, the Creator and Sustainer of all that exists, tells us the answer in His Quran..."
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"Say  - 'Tell me then, the things you call upon besides Allah - If Allah had intended harm to me, could they (these things) remove HIs harm? Or had He (Allah) intended Mercy for me, could they withhold HIs Mercy?"
Say, "Enough for me is Allah; in Him do those who trust (true believers) must put their trust."
Quran 39:38

 Translation of meaning - "And if you asked them, "Who created the heavens and the earth?" they would surely say, " Allah ."
Say, "Then have you considered what you call upon for help besides Allah ? If Allah intended me harm, are they removers of His harm; or if He intended me mercy, are they withholders of His mercy?" Say, "Sufficient for me is Allah ; upon Him [alone] rely the [wise] reliers."

In this verse Allah, The Most Glorified, Most High commands His Prophet Muhammad  to reject those powerless, graven images worshipped by the polytheists, which can neither remove any harm which might befall a person by Allah's Decree, nor prevent any sustenance or blessings which might come to a person from Him. Then He commands him to place his trust in Allah, for He is Sufficient to bring benefit or prevent harm for all who sincerely depend upon Him.
Benefits Derived From This Verse

1. The obligation to reject the abominable.
2. The falseness of idol-worship.
3. That the removal of harm and the bringing of benefits is only from Allah.
4. The obligation to trust in Allah and depend upon Him, free from dependence upon any other, and this does not conflict with the requirement to take whatever legal measures one can in order to achieve one's objectives.

Relevance of This Verse to the Subject of the Chapter
That it proves that protection from harm is only from Allah , and so entrusting oneself to any other protector - such as rings, threads etc. is an act of Shirk.*
 
*It is reported by Ibn Abi Hatim, on the authority of Huzaifah (ra) that he saw a man with a thread in his hand to protect him from fever; he broke it and recited the Words of Allah (ra), Most High: " And most of them do not believe in Allah, except that they associate partners with Him" (Qur'an 12:106)

Hudhaifah visited a sick man and found him wearing a thread on his wrist; and when he asked the man what was the purpose of it, he told him that it was a protection from fever, at which Hudhaifah broke it declaring it to be Shirk; and as proof of this, he recited the Words of Allah , Most High: " And most of them do not believe in Allah, except that they associate partners with Him" - and the meaning of this verse is that many people believe in Allah , but adulterate their belief with Shirk.
It is also reported by Ahmad on the authority of `Uqbah Ibn A`mir (ra) in a marfoo' 6 form: "Whoever wore a tamemah (Talisman or amulet). Allah will never fulfill his wishes, and whoever wears a wada'ah (A sea-shell resembling an oyster shell). Allah will never grant him peace and tranquility." - and in another narration of Ahmad: "Whoever wears a tamemah has committed an act of Shirk."

`Uqbah Ibn A`mir (ra) informs us in this Hadith that the Messenger of Allah, peace be upon him,  supplicated against every person who wears a talisman or a seashell, believing that it will benefit him without Allah , that Allah will not allow him to achieve any of his goals nor fulfill his wishes or dreams; rather, He will prevent him from ever finding security and tranquility; and he also informed us that any such action is false; indeed, in another narration, he informed us that the tamemah is a form of Shirk because its perpetrator believes that it will benefit him without Allah .

It is reported on the authority of Umran Ibn Husain (ra), that Allah's Messenger  saw a man with a brass ring in his hand, and he asked him: "What is this?" He replied: "It is for protection from al-waahinah (rheumatism)." The Prophet  answered: "Remove it at once, for verily, it will not increase you except in weakness, and were you to die whilst wearing it, you would never be successful." (Narrated by Ahmad, with an acceptable Sanad)

Umran Ibn Husain informs us in this Hadith that the Prophet  saw a man with a brass ring in his hand, and he asked him for what purpose he was wearing it. The man answered that he was wearing it to protect him from illness, at which the Prophet  ordered him to remove it and informed him that it would only cause him to become weak and not protect him from illness at all, and that should he die while wearing it and believing in its power to protect him, he would not succeed in the Hereafter, nor would he know eternal bliss.

We ask Allah to keep us away from anything and everything associated with "shirk", ameen.