The photograph is of an "electron tree" trapped inside a block of clear acrylic material. "Trees" are produced by bombarding the face of an acrylic block with electrons, produced in this case by high power AECL IMPELA accelerator. The trapped energy is then released by a sharp blow with a grounded pointed instrument, the rapid discharge leaving a permanent trail inside the acrylic block.
The block was photograph using polarized transmitted light and a second polarizer on the camera lens to improve the image contrast. The pseudo-rocks are in fact large chips out of the acrylic block. The perfect specimens are reserved for use as gifts to AECL Accelerator Branch retirees!\