Neat trick they’re using to fill it with epoxy with a specific refractive index to render it transparent. I’m familiar with that from working with glass… different glasses have varying indexes of refraction, and so do liquids. For artistic purposes, it affects how brilliant they look when cut and faceted - for instance lead glass sparkles a lot more than borosilicate, which is why one reason used for the well known traditional style of cut and polished decorative vases and cups. If you put a piece of clear glass into a beaker filled with water, you can typically see the outline of the glass underwater. However, if you tweak the refractive index of the water to match the glass, the glass is rendered invisible. This isn’t super useful but can be employed to distinguish types of glass or create niche artistic effects.