optically transparent FFF 3D prints just were not possible. The process itself worked against this. Then Colorfabb wrote a blog post asserting that it was possible, with a special filament. So I knew I had to give this "HT" stuff a try.
What is Colorfabb HT? I have no idea. Even the name "HT" doesn't describe anything other than the properties of the blend. Whatever it is, it's designed to print and tolerate High Temperatures. But apparently it has another property, that causes it to continue to flow a little after printing and fill in the air holes left between the lines inherent in the FFF process. Normally this would be a bad thing, but apparently it wasn't so bad that they needed to throw out HT all together and produced this marvelous property that we are exploiting to make the coolest set of dice ever.
Download and print your own embedded transparent skull dice, if you dare: https://pinshape.com/items/34252-3d-printed-embedded-skull-die-for-transparent-3d-printing