A ground-glass nodule looks like ground glass in the lung on an X-ray. A nodule is any spot in the lung that is smaller than 3 centimeters in diameter. Ground-glass nodules are more likely to be cancerous, according to About.com.
Lung nodules are not always cancerous, and they are actually quite common. They show up on approximately 1 chest X-ray out of every 500 and 1 CT scan out of every 100. If multiple nodules show up, or if any ground-glass nodules show up, the doctor is likely to recommend other tests to look for cancer that may have spread to the lungs, reports About.com.