Vein

In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation.

Fibrous mineral veins generally grow by precipitation from supersaturated aqueous solutions.The force of crystallization as a function of the degree of solution supersaturation (ratio of the actual concentration to the concentration in a normal saturated solution) and the partial molar volume change of the precipitated mineral.





http://en.wikipedia.org/wiki/Vein_%28geology%29