The Concuspidor's "Lucky" Dice:

Numbers in the range 2 - 12

[Note for mathematicians: a "bell curve" probability is produced using these dice, although its exact nature depends on who is throwing them]

Black die: sufficiently dextrous fingertips can flip the white spots over on their tiny axles to show their black sides. In this way low numbers can be anticipated as the die is handed to a competitor.

White die: no amount of dexterity can alter the utterly conventional spots on the die. In this way fair numbers can be anticipated should a die ever need to be inspected by a rudely untrusting competitor.