**Core Concept:**
Gallstones are solid, non-radiopaque formations within the gallbladder. They can be classified into two types: cholesterol stones and pigment stones. Cholesterol stones are the most common type and are formed due to the precipitation of cholesterol crystals from the bile. On the other hand, pigment stones are formed due to the precipitation of bilirubin, which is predominantly seen in patients with hemolytic disorders or liver diseases.

**Why the Correct Answer is Right:**
Cholesterol stones are the most common type of gallstones and account for about 80-90% of all gallstones. They are typically radiolucent (not radio opaque), meaning they do not show up on X-rays. This is due to the absence of calcium salts, which are essential for the stones to be radiopaque.

**Why Each Wrong Option is Incorrect:**
Option A and D are incorrect because they mention "always" which is not true for cholesterol stones. Cholesterol stones do not always appear radioopaque.

Option B is incorrect as it states that pigment stones are always radioopaque. Pigment stones, however, are predominantly formed due to the precipitation of bilirubin and are typically radiolucent.

**Why the Correct Answer is Right:**
The correct answer (C) states that "most" gallstones are radioopaque. This is accurate as a significant portion of gallstones, particularly those composed of calcium salts, are radiopaque. Additionally, radiopacity is dependent on the presence of calcium salts.

**Why Each Wrong Option is Incorrect:**
Option A is incorrect as it mentions "always", which is not accurate for gallstones. Gallstones can be of different compositions and not all are radioopaque.

Option B is incorrect because it mentions "pigment stones", which are radiolucent, not radioopaque.

Option D is incorrect as it states "calcium stones", but calcium stones can still vary in their radiopacity based on their composition.

**Clinical Pearl:**
The presence of radiopacity in a patient with abdominal pain and suspected gallstones is an important clue for the presence of calcium stones. However, radiopacity does not confirm the presence of gallstones as radiolucent stones can also cause similar symptoms. A comprehensive evaluation involving imaging studies like ultrasound, MRI, or CT is essential for definitive diagnosis and treatment planning.