What substance doES not allow x-rays to pass through it?

Lead

X-rays are a type of electromagnetic radiation, similar to visible light but with much shorter wavelengths. This means that x-rays can pass through many objects, such as flesh, wood, and plastic. However, some materials, such as lead, are so dense that they effectively block x-rays. This is why lead is often used as a shielding material for x-ray machines, such as those used in dental offices and hospitals.