Surprisingly, I actually ran into this last semester during my capstone project. I am trying to make a transparent skull to study the effects of traumatic brain injuries and in order to do that I have to get proper geometry of the skull. We used MRI files and converted that to a 3D model. One of things we have to look into are materials that are similar, mechanically, to skull but also transparent. After googling several we came up with a few ideas, seemingly easy. At the time we thought we could choose any material that met the requirements. However, we came to realize the feasibility of using such materials. To give you an example, one of the choices we came up with was PEEK (poly ether ether ketone). It is used in craniomaxillofacial implants and we thought it would be perfect to use this material. Feasibility wise, not so much. It is very very expensive and would require an industrial grade foundry to mold. Something so simple as choosing a material became one of our critical paths because we had to ensure we chose a material that met our requirements but was also feasible.