By Bill Hubbard, Palomar Gem, April, 1999
When strolling past the display cases at a gem and mineral show, spectacular crystal specimens on exhibit often blow our minds. We cannot help but ask "Where on earth did they find that?" Actually crystals are ubiquitous. Just about every solid we see when we look around us was, is, or will be crystalline and that includes ourselves. Our bodies are over 80% water and probably every molecule of the water has been part of a six-pointed crystalline snowflake floating down from the wintry skies countless times. In fact, while ice is not normally thought of as a mineral, it is one.
So here is one of the secrets to big fabulous crystals like you see in the display cases. Great crystals require an open space, a cavity, room to grow.
Most rocks are crystalline. A few, such as obsidian, exist temporarily in non-crystalline forms. However in the average rock, such as granite, the crystals are all tightly jammed together and have distorted each others shapes. When we look at a piece granite, the speckled pattern is caused by differing crystals that make up the rock. Typically the gray is quartz, the pink is feldspar and the black is biotite. A mineralogist looking through his petrologic microscope at a polished thin section of a rock specimen can identify all the various crystal grains jam packed into the rock.
The formation of crystals in a rock cavity requires two separate steps: one, the initial creation of the cavity, and two, the growth of the crystals... Each of these steps has several sub elements. In step two, the initial requirement is for a nuclei or seed before the crystal will start growing. These two processes - cavity creation and crystal - growth can operate almost simultaneously. Most often they are independent of each other, operating sequentially over time.
Cavities in rocks are formed by a number of methods. For example, during faulting as a result of plate tectonic activity, uneven rock masses slide by each other creating empty spaces. These are called fissure veins. Gas bubbles form voids in volcanic flows. Caverns are examples of solutions dissolving rock and forming open spaces. Contractions related to the cooling of an intruding magma create numerous voids.
Now two fundamentals about minerals and crystals. A mineral is defined in as "a naturally occurring homogenous solid within a definite (but generally not fixed) chemical composition and highly ordered atomic arrangement." Crystals are minerals wherein the "constituent atoms are organized in a regular geometric pattern that repeats in three dimensions to form a solid body." (Gems & Crystals).
So how do cavities become filled with crystals? The most typical mechanism is migrating hydrothermal solutions under pressure and saturated with dissolved minerals. In other words it is pressurized hot "hard water" similar to that which builds up mineral deposits in hot water heaters.
Very large cavities enable grand specimens to form. The atoms in the mineral rich solutions organize themselves from stochastic randomness into symmetrical, ordered, geometrical patterns. Each mineral species has its own unique atomic arrangement. Presence of necessary chemical components and time are essential for deposition of all crystals. All things being equal, it takes longer to grow a large crystal than a small one. If you ask "How long?" We will just have to say "long enough."
But back to the other facets of crystal growth. There can be too much of a good thing. Should a rich fluid medium persist in the cavity for too long a time, the crystals get too big and grow together in an interlocking mass. In such a situation, discrete crystal faces and potentially prized specimens disappear.
An important thing to remember in crystal hunting is that there are many more small cavities than large ones. These small cavities are usually called vugs. Vugs are more often than not lined with crystals, frequently perfect crystals. To enjoy this aspect of rockhounding, we suggest getting yourself a hand lens if you don't already have one and learn how to use it.
Vugs are the stomping grounds of micro-mount enthusiasts. Some wonderful crystals only form in small cavities. But we hinted at the beginning we would tell you where to find big crystals, so why are we talking about vugs and hand lenses? Well, we have discovered that as we get more acquainted with small crystals in the field, large ones suddenly start to "jump out to bite us."
By the way, crystals lying around on the ground are pure accidents. They have been eroded from underlying rock or washed in from some nearby source. For good, big crystals you need to be looking at bedrock or material that came from bedrock. We do not advocate that you go down any mine shafts, but there are a lot of old mining areas that have surface cuts, pits, trenches and dumps. Then there are road cuts.
Finding crystals in the field is aided and abetted by owning a few to start with. Specimens in the matrix are especially good teachers. Places other than a field trip to acquire crystals are gem and mineral shows, rock shops, tail gating rendezvous, trading with other rockhounds and decorative rock yards. We found such a yard in Whitewater (near Palm Springs) that sells huge selenite crystals in small lots (or by the ton) at bulk prices. Another way to find crystal locations are the numerous guide books on collecting sites.