Option 1: X-rays are not used for cutting and welding of metals. X-rays are a form of electromagnetic radiation and do not have the physical properties required for cutting or welding.

Option 2: X-rays can be used to detect certain heart conditions, such as blocked arteries or heart valve abnormalities. However, this is not the primary use of X-rays in medical imaging. Other imaging techniques like echocardiography or cardiac MRI may be more commonly used for detecting heart diseases.

Option 3: This is the correct answer. X-rays can be used to detect defects in precious stones and diamonds. X-ray imaging can reveal internal flaws, cracks, or inclusions that may affect the quality or value of the gemstone.

Option 4: X-rays are not used to detect gold under the earth. Other techniques like ground-penetrating radar or seismic surveys are used for locating underground deposits of gold or other minerals.

Overall, option 3 is the correct choice as X-rays are commonly used in the jewelry industry to detect defects in gemstones.