Take six bears, place three on each side of the scale.

If the two groups are of equal weights then the diamond must be one of the two remaining bears: balance the two remaining bears and you'll know which has the diamond (the heavier one).

If they are of different weights, then take two of the bears from the heavy side and weight them. If they are of equal weights, then the remaining bear from the original heavy side has the diamond. If they are of different weights, then the heavy bear has the diamond.