**Core Concept**
The tarnished layer of silver amalgam is a result of the oxidation of the mercury in the amalgam, leading to the formation of a compound that causes discoloration. This process is an example of a chemical reaction between the mercury and oxygen in the oral environment.

**Why the Correct Answer is Right**
The tarnished layer is primarily composed of silver sulfide (Ag2S) and mercury sulfide (HgS), which are formed through the reaction of the mercury in the amalgam with the sulfur compounds present in the saliva and oral bacteria. This reaction results in the formation of a black or dark gray layer on the surface of the amalgam restoration.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the tarnished layer is not primarily composed of silver oxide (Ag2O).
**Option B:** Incorrect because while silver sulfide (Ag2S) is a component of the tarnished layer, it is not the primary component.
**Option C:** Incorrect because the tarnished layer is not primarily composed of mercury oxide (HgO).

**Clinical Pearl / High-Yield Fact**
It is essential to note that the tarnished layer does not affect the strength or durability of the amalgam restoration, but it can make the restoration more susceptible to corrosion and wear.

**Correct Answer: C. Mercury sulfide (HgS).**