This is pure speculation based on how dice are manufactured, but it’s entirely possible that transparent or translucent style dice may technically roll more true than opaque dice.
I was talking with some dice manufacturers who told me that the plastic used to manufacture dice is actually clear by default, and opaque dice have coloring added. This seemed odd to me, and I asked why basic opaque dice were vastly cheaper than, say, clear dice that wouldn’t require additional additives.
The short answer: opaque dice have air bubbles inside them.
Why it Happens
Air bubbles can form inside the dice during the plastic injection molding process when the dice cool too quickly. With opaque dice, since the air bubbles can’t be seen, dice manufacturers deliberately cool the dice too quickly, which allows them to run more plastic through the molds per hour — thus the cost of the dice is cheaper.
Because the air bubbles form irregularly and in different sizes, an opaque die may well have one side that is lighter than another side, which could cause the die to favor one side.
Now it’s also possible that the premium opaque dice (like the one pictured above) don’t have this issue: they are vastly more expensive than your basic opaque dice, and indeed are more expensive than basic translucent dice. Is this because they aren’t pushing them through the molds as fast as the basic opaques, or just a factor of the difficulty in combining multiple colored plastic in a consistent way?
Does it Matter?
It’s worth stressing that I don’t know of any tests that have been done that demonstrate whether or not air bubbles or density differences actually affect the rolling of the dice. In our dice randomness test, we definitely saw that irregular surfaces on the face of the die affected the roll, but we certainly didn’t test anything with irregular densities.
Interestingly in that test we compared an opaque Chessex die against a translucent GameScience die: the GameScience die rolled marginally more true (other than the side with the sprue), but was it because of the manufacturing method, or was it because the opaque Chessex die had air bubbles affecting the roll?
If I had to guess, I do not think that air bubbles would make a measurable difference. If they did, that difference would certainly be miniscule and irrelevant to RPG gaming (much like we saw in Chessex vs GameScience). But until someone does a giant rolling test of opaque vs translucent dice, the possibility remains that opaque dice roll less true.