These pictures show the effect of adding copper (II) salts to a saturated solution of Rochelle Salt prior to recrystallization.

The effect is quite dramatic in that not only is copper included in the crystal lattice to impart varying shades of blue but it also has an effect on the crystal shape (or "habit"). These two effects are concentration dependent. Note the predominant diamond shape in some, but different shapes in others. Recall that snowflakes can come in different shapes too, even though thew are all made from the same substance.

This effect on crystal habit was first noted by JH Thorpe and HE Buckley in their article "Morphological and Dielectric Studies of some Crystals of the Rochelle Salt Type", which appeared in Acta Cryst. 2:333-337 (1949).

You may also view a copper-contaminated multiple crystal or a "pure" Rochelle Salt crystal.