Alluvial vs. Primary
If you're new to buying rough-like we all were at some point- then it's important to know the difference between "primary" and "alluvial."
The mineral solutions that come together and produce gem rough/crystals occur at precise locations around the globe. Some of these locations are known and still others are waiting to be discovered. When the precise location where minerals have grown is found, it is called the "primary" deposit. If you've seen some of my videos exploring tanzanite or rhodolite mines where I am going down tunnels, you have seen primary deposits - where the miners are chipping away to extract the jagged pieces of rough.
Other times, erosion (either mechanical or chemical) occurs and the precise location where the minerals grew is disturbed. The rough breaks free of the host rock and begins to travel, sometimes great distances, from the primary source.
If we are lucky, the traveling rough makes its way to a river-or a river makes its way to the primary deposit. The action of the moving water begins to work its magic by smoothing out the jagged edges of the rough- these are called "alluvial" stones or deposits. These stones are found by miners when they either wade into the river and scoop up the gravel under water looking for stones, or they dig into an ancient and dried out river bed which occurs when the river has changed its course or gone extinct.
In general, given a choice between a "primary" or "alluvial" piece of rough, faceters will usually chose alluvial stones. They prefer them for a couple of reasons. One is yield - the return a cutter gets from a piece of rough. The closer the shape of the rough is to the finished gem, the higher the yield and therefore the higher return on investment.
Another reason is because alluvial stones tend to be cleaner stones. Most inclusions are weak spots in the rough's structure. The tumbling and tossing around in a river will usually result in these weak spots being knocked off...and if we are lucky, leaving behind a flawless nodule.
This is not to say that "primary" deposits are less than "alluvial" deposits. No way! Emerald, Tsavorite, Tanzanite, many sapphires, Mahenge spinels, ad infinitum come from primary deposits and are some of the most valuable gems on the earth!
Now that I have described the difference between a primary and alluvial piece of rough, it should be quite obvious which is which in the photo!
I'm not a geologist (I'm a geographer) and while some of this can be explained with fancier scientific language, I hope the novice will find this information helpful