Taking that a bit further, the difficulty of opening the boxes is proportional to the square of the box number, and the overall chance of finding a diamond (taking into account how hard it is to open the box as well as the likelihood of a given box containing a diamond) is inversely proportional approximately to the cube of the box number times the logarithm of the box number. Diamonds in higher numbered boxes are much harder to find. You really don't want to miss the easy ones, ever.
The allure of progressing twice as fast is obvious, but the penalty for missing a prime is tremendous.