The edge of super-saturationCrystals grow best when the solution they are in is on the edge of super-saturation.
If a solution is too saturated spontaneous seeding can occur, and crystal growth is rapid, but haphazzard.
On the other hand, if a solution is insufficiently saturated, crystals dissolve, rather than form.
Migration to the edge of super-saturationFortunately, natural forces conspire to take naturally-ocurring bodies of water to the edge of super-saturation.
In particular, finely-ground rocks are continuously added by the processes of erosion - and then any excess of crystallisable material is rapidly removed by crystallisation.
The larger the body of water in question the better this process works - and the less likely it is to be susceptable to being caught out by local fluctuations.
When a solution is on the edge of super-saturated, crystals are most likely to develop without flaws. The process depends on the dissolving point of crystal formations being lower if they are not properly aligned with the rest of the crystal.
The oceans today are super-saturated with respect to various mineral compounds.
For example the rate of coral reef growth is known to be proportional to the extent of the calcium carbonate super-saturation.
Almost any lake can become super-saturated through the process of evaporation - which can cause extreme concentrartions of minerals, such as those that occur in the Dead Sea.