Microscopically, it appears as small rounded grains that are uniform in size making it readily distinguishable from Scheele's green. At high magnification, the grains are radial in structure and a dark spot can be seen in the center. It can be identified chemically by the stannous chloride test described for Scheele's green and has the same reaction for arsenic. In potassium hydroxide, it turns into an ochre color and in weak sulfuric acid it dissolves and turns blue.
Copper (II) - acetoarsenite
Cu(CH3COO)2 · 3 Cu(AsO2)2
alpha = 1.71-2, beta = 1.77-8
Green or blue-green
The reason transparency and clarity are factored together is that flaws will affect the amount of light that passes through the stone. You are looking for a gem through which light passes easily without turning cloudy, milky or hazy.
To examine an emerald for clarity and transparency, follow these easy steps:
1. Wipe the stone with a soft cloth and view it under fluorescent, overhead lighting both with the naked eye and with the assistance of a 10X magnifying loupe.
2. Look at the stone from all angles making sure to let light reflect off the surface. Such reflection can reveal dangerous cracks
3. Remember that just because a particular emerald might exhibit perfect color, if it is less than semi-transparent bordering on opaque, the color factor is moot.
4. When shopping for top quality gemstones, it is always best to purchase loose stones and have them set yourself. Prongs and other settings can hide flaws and make close examination very difficult
5. Keep in mind, the cut of the stone effects clarity. Always compare similar cuts with one another
6. After observing the stone under overhead lighting, ask to see the stone under diffused lighting shining up diagonally through the bottom of the stone. This reveals some inclusions you may miss with overhead lighting.
7. Also remember that lighter stones will exhibit better clarity than darker stones. This does not mean the lighter stone is higher quality, so compare darker stones with darker stones.