**Core Concept**
Radio-opaque gallstones are formed due to the presence of calcium salts, particularly calcium carbonate or calcium phosphate, which are visible on plain X-rays. This is in contrast to cholesterol gallstones, which are not radiopaque. The formation of radio-opaque gallstones is often associated with chronic biliary tract disease, such as gallbladder cancer or primary sclerosing cholangitis.

**Why the Correct Answer is Right**
Radio-opaque gallstones contain calcium salts that make them visible on X-rays. This is because calcium is a dense element that absorbs X-ray energy, making the stones appear as radiopaque structures on the X-ray image. The formation of these stones is often related to chronic inflammation and bile stasis in the gallbladder, which can lead to the precipitation of calcium salts.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because radio-opaque gallstones are not formed primarily by bilirubin or its derivatives, which are more commonly associated with pigment gallstones.
**Option B:** This option is incorrect because radio-opaque gallstones are not typically formed by cholesterol crystals, which are the main component of cholesterol gallstones.
**Option C:** This option is incorrect because radio-opaque gallstones are not primarily composed of bile salts, which are more commonly associated with bile duct obstruction.

**Clinical Pearl / High-Yield Fact**
Radio-opaque gallstones are often associated with chronic biliary tract disease and can be a marker for underlying malignancy or chronic inflammation. It's essential to consider the patient's clinical presentation and history when interpreting the presence of radio-opaque gallstones.

**Correct Answer: B. Cholesterol crystals are not radio-opaque.