**Core Concept**
Radio-opaque stones are formed in the gallbladder or urinary system due to the presence of radiodense materials. These stones can be made of calcium salts, oxalate, or other substances that can be visualized on radiographic imaging.

**Why the Correct Answer is Right**
Radio-opaque stones typically contain calcium salts, such as calcium oxalate or calcium phosphate, which are radiodense and can be seen on X-rays. The formation of these stones is often related to supersaturation of the bile or urine with calcium salts, leading to their precipitation and aggregation.

**Why Each Wrong Option is Incorrect**
**Option A:**
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**Clinical Pearl / High-Yield Fact**
To remember the common types of radio-opaque stones, recall the acronym "COP" for Calcium Oxalate and Phosphate stones, which are the most common types of radio-opaque stones.

**Correct Answer:**
Please provide the complete question, including options A, B, and C.