aquamarine.com

Talk to any gemologist or mineralogist long enough and eventually you will hear them launch into a detailed discussion of pegmatites—detailed because when it comes to pegmatites, it can’t be any other way. These mineral bearing slabs are complicated to say the least, yet understanding them is important to understanding gemology.

If you think that a pegmatite might look like something you’d find in a cave, you could be right, but they don’t have to be in caves. They can be sitting on top or near the top of the ground. What characterizes these rocks is the way they were formed under pressure and heat, yielding course crystals in granite or silica. In other words, that granite countertop in your kitchen probably started out as a pegmatite.

Also, pegmatites are commonly found in mountains and can have almost any shape or size. Some may be irregularly shaped, while others may form tables. Similarly, there is no standard size, given that they can range from a few feet to a fee thousand feet in thickness.
When pegmatites are exposed to elements, such as wind and rain, the resulting erosion produces alluvial deposits in streams and rivers. This is why some miners spend their time sifting through pebbles that have been washed to the surface. Compare these to the 49ers who panned for gold in the famed California Gold Rush.

To find aquamarine, start looking for pegmatites. Obviously, not every pegmatite contains this gem, but pegmatites are their primary hiding place, which at least partially explains why some of the finest aquamarines come from Minas Gerais, Brazil, a region rich in mineral bearing pegmatites. Because the mineral deposits in these pegmatites are so rich, it sometimes seems that gemstones like aquamarine have turned up all over the place in Brazil. Folks have found them alongside the road in ditches, in streams and in shallow holes. Who knows where the next great find could be, but one thing is for sure. Most likely it started somewhere in a pegmatite.