Cross‐sectional information on low electron density materials can be obtained by probing a sample with a 60 keV coherent synchrotron x‐ray beam in an in‐line holography setup. Such objects are practically transparent to high energy x rays and create a phase shift of the wave front only. Images of a 100 μm diameter boron fiber were recorded in the extreme near field region, where contrast occurs only at interfaces between regions with different decrements of refractive index. Theoretical simulations are in good agreement with the measured intensities. In a tomographic reconstruction the 15 μm diameter core of the fiber is clearly visible, demonstrating the possibility of reconstructing three dimensional interfaces between low density materials.