What is preferential dissolution?

First of all, let’s talk about growth: the chemical elements that are available during the growth of a crystal might change slightly over time. Sometimes, it doesn’t show unless you do actual chemical analyses, and notice slight variation in minor elements (changes in the 1% level or less). Some other times, you can actually see it, as the color of the mineral changes, such as for these two tourmalines.

Zoned liddicoatite tourmaline (22 x 17 x 0.7 cm); Anjanabonoina mine, Antsirabe,

Antananarivo, Madagascar.

Bi-color tourmaline (10.7 x 3.7 x 3 cm); Kunar, Afghanistan (donor: Savinar).

Click to read more about preferential differentiation on different gems!

In the case below, the beryl has been naturally etched, dissolving the surface of the crystal, but preferentially one layer inside the crystal. This is most likely explained by a small difference in the chemical composition or amount of defects of the beryl, making this layer more prone to dissolution than the rest of the crystal.

Beryl, variety aquamarine from Minas Gerais, Brazil. Dissolution along one layer made this aquamarine a fun toy!

To the opposite, the aquamarine below didn't show any preferential areas for the dissolution to happen. The original crystal faces are hardly recognizable.

Beryl, variety aquamarine from Minas Gerais, Brazil (donor: Savinar). Spectacular dissolution!

Another example of preferential dissolution is the one occurring in some pink and brown diamonds. As explained in this blog post, the pink color is restricted to some pink thin lamellae. These lamellae are extremely rich in defects, and preferential dissolution along them might occur. For diamonds, the typical sign of dissolution is the presence of "trigons" (triangular-like features). In the two pictures below, you can see that the trigons preferentially occur along the pink lamellae, which are the weakest areas of these diamonds. 

Images of two pink diamonds taken by reflection. Etched features (trigons) are aligned along lines, that are in fact the intersection of the pink lamellae with the surface of the diamond.

One of my favorite specimen showing preferential specimen is this fluorite, on display at the Smithsonian Institution:

These are just a few examples, don't hesitate to share yours!