Answer:
All materials with atoms containing electrons (i.e. just about all materials that we ever encounter) have the properties of a diamagnet. A naive picture of what's happening is that the orbiting electrons in the atoms create a magnetic field in response to an external magnetic field that repels the external field.
The property of diamagnetism is universal - it even exists in paramagnets such as Samarium Cobalt magnets. The fact that we don't notice it much is that it is weak - about a thousand times weaker than strength of a paramagnetic material.