We report a novel method for developing gelatin-based phantom materials for transmission x-ray imaging with high stability at room temperature and tunable x-ray attenuation properties. This is achieved by efficiently cross-linking gelatin in a glycerin solution with only 10% water by volume and systematically decreasing their x-ray attenuation coefficients by doping with microbubbles that are originally designed to be used as lightweight additives for paints and crack fillers. For demonstration, we mimic breast glandular and adipose tissues by using such gelatin materials and also study the feasibility of 3D printing them based on the extrusion-based technique. Results from x-ray spectroscopy (15–45 keV) show the materials to have stable x-ray attenuation properties of glandular and adipose tissues over a period of two months. Micro-CT analysis of independently prepared samples shows the materials to be uniform and easy to reproduce with minimum variability in attenuation values. These materials can be used to 3D print realistic phantoms that mimic x-ray properties of various biological tissues.