A glass die, around say, a half-inch square, has transparent faces, each fashioned from glass/crystal with polarisation patterns oriented at different angles.
Inside the die, suspended by an arrangement of inset magnets is a carefully balanced, smaller die, similarly constructed, and inside that
a further one, and inside that a further one etc.
Rolling the die tumbles each of the interior dice, each of which will 'snap' into a particular position due to the gentle magnetic suspension.
The orientation of the polarising surfaces will be set up so as to ensure that they never completely block out any incoming light.
In any given roll, for each different internal die, faces will either line up exactly with the outer face, or land at an angle unique to that particular die. Internal dice have polarization orientated at a value dependent on the total number of internal dice.
For example, a die containing 7 internal dice, will need to separate each surface by an angle of at least 11.25°, while one containing only 3 internal dice, can have internal rotational differences of around 22°
To read the die, it is placed into a graduated light and the amount of light at various polarisations that comes out the other side at each different orientation is measured.
A clarity of 100% will be considered a 'roll' of 255 (or whatever the maximum number of internal die that can be fitted into the construction) and the amount of light at each rotational point will reveal the state of each of the embedded dice, which can be represented as a binary on or off. Thus, a die with 7 internal, spectral' dice should be able to produce random numbers between 0 and 255 - or 1 and 256)
What would be cool (but I've not yet worked it out) would be a method of shining light through the die, and having a readable value projected onto the wall or ceiling, given an appropriate light-source.
An eight-bit die might pose some construction difficulties (due to the exponential size decrease), but perhaps a smaller-bit (maybe a 4-bit device, counting up to 16) could be made to do something like this.