Plated of quartz, etched internally via focused femtosecond pulsed-laser to create tiny voids or just changes in crystal lattice that encode bits as changes in the polarization of light passing back through the glass.

It's been studied in various ways for a while now.

https://en.wikipedia.org/wiki/5D_optical_data_storage

https://www.youtube.com/watch?v=V7L_wdEuQXs