X-ray diffraction has been used to characterize the amorphous phase present in a series of radiation-damaged natural zircons with radiation doses ranging from 0.06 to 16 × 10¹⁸ -decay events g^-1 . The fraction of amorphous material present in each of the samples studied has been determined, and its dependence on the radiation dose has been calibrated. Direct determination of the amorphous fraction confirms that amorphization in natural zircon occurs as a consequence of the direct impact within cascades caused by -recoil nuclei. These results are not consistent with the commonly accepted double-overlap model of damage accumulation. The volume swelling of amorphous regions changes as a function of dose. Thus, the density of amorphous regions depends on the degree of damage up to a certain point (i.e. 8 × 10¹⁸ -decay events g^-1 ), unlike in previous models for which a constant value independent of the radiation dose was assumed.