I have two thoughts. 1)Are you sure it wasn't darker to begin with? Diamonds have different brightness.

2)It may not be a diamond, whereas the others are. A paste stone may have replaced a lost one, sometime in the past. If it was your grandmother's, they may not have had good fakes. And if you've immersed it in something that wouldn't darken the actual diamonds, it may have had an adverse effect on a glass stone.