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Beryllium has other fancy applications. It is transparent to x-rays, so it's used in the windows of x-ray tubes, which need to be strong enough to hold a perfect vacuum, yet thin enough to let the delicate x-rays out.

The quote is from the book The Elements by Theodore Gray. Why is beryllium transparent to x-rays?

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    Graphene has now also been used for x-ray windows, with better transmission in the range 0.1 - 3 keV: http://arxiv.org/abs/1503.03327 – Andre Holzner Apr 12 '15 at 21:02

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The amount of X-rays absorbed by an element depends on the size of its atoms (its absorption cross-section, specifically, as affected by the size of core orbitals that contain electrons that can be excited by X-ray absorption, and the number of electrons in those orbitals) and how many atoms are packed into a given volume. Big atoms that are close together can absorb more X-rays than small atoms that are far apart.

Beryllium has very small atoms (it has a very small electronic core, with only two electrons in it), and very low density, so it will absorb relatively few X-rays compared to other materials.

According to this page, 37 % of X-rays with a wavelength of 0.56 nm will be absorbed by beryllium window that is 23.5 mm thick. If the window were made of aluminum, it would only have to be 1.4 mm thick to absorb the same amount of X-rays, and if it were made of lead it would only have to be 13 μm thick.

Fred Senese
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    The notion of "size" here is hooey. Beryllium has a very low Z number so its electrons are relatively loosely bound. X-rays will in general have an energy great enough to cause some sort of electronic excited state, but as the photons increase in energy beyond what is needed, then the probability of an interaction between the photon and the electron goes down. The other factor here of course is that atoms of beryllium have relatively few electrons per atom of course. – MaxW Nov 15 '16 at 20:48