Quartz is so hard that it will easily make a deep scratch in glass. Even if you press hard, a meteorite will at best only make a weak scratch mark. (Use a sharp edge of the rock.) In the photo above, the top scratch was made with a quartz crystal. The next mark was made with a piece of chert, which is another form of quartz. The two scratches at the bottom were made with ordinary chondrites (click on image for enlargement of bottom portion).
tomography. any method that produces images of single tissue planes. In conventional radiology, tomographic images (body-section radiographs) are produced by motion of the x-ray tube and film or by motion of the patient that blurs the image except in a single plane.
Aetna considers virtual colonoscopy using computed tomography (CT colonography) performed every 5 years a medically necessary preventive service for colorectal cancer screening of average-risk asymptomatic persons 50 years of age or older.