B. Look Inside
One of the most interesting and useful elements of gemology is the study of inclusions. As you begin examining gems, you will encounter quite a variety of internal paraphernalia. The definition of an inclusion is anything that will affect the flow of light. While this sounds negative, you will find that some inclusions are beautiful.

Others are nothing short of incredible, as you learn what they are and how they got to be inside the gem. One of the easiest categories of inclusions to identify are fractures. Emeralds almost always have internal fractures, while they are uncommon in most gems. Tiny, internal fractures that don't reach the surface will have minimal effect on the durability of the gem. The fact that they have gone through the cutting process without damage attests to that. Larger fractures, or those close to a thin edge, are significant.

If a fracture reaches the surface of the gem, it has the ability to absorb liquids. Over time this can absorb dirt and skin oils, loosing some of its brilliance.

In Emeralds, this feature is used to absorb oil or other substances with the same RI as the gem. This makes them much less visible. To see them, you need to look very closely and get the light on them from just the right angle. If you see a multicolored radiance, like an oil slick, you have spotted a filled fracture.
Sometimes a fracture will stand out, interrupting the flow of light. Other times the fracture is pressed so tightly together that light will flow right through the fracture. This type will show a bright, multicolored flash when light strikes it from the correct angle.

Most fractures have at least a slight bit of curvature in them. Look carefully to see this. If you encounter one that is perfectly straight, that is most likely a cleavage fracture. Gems have cleavage planes that are much like the grain of wood. Along the cleavage plane the gem splits easily. Finding a cleavage fracture in a gem is a sign of significant weakness.

The other most common type of inclusion are other minerals. These can be small bits of debris or miniature, whole crystals. Afghani peridot sometimes has clusters of tourmaline that look like flies. Spinel can be included with tiny spinel crystals arranged in a plane. These are often spectacular!
Some mineral inclusions are opaque. Their size and number have to do with how much they will interfere with the passage of light. Black, "carbon" spots in diamonds are a common example of this.

Other inclusions are transparent. How visible they are depends on their RI. If they are significantly different than the surrounding crystal, they will jump right out at you. In other cases, like spinel in spinel, they are nearly invisible. To see them you need to have a dark background behind the gem and light coming in from the sides.

"Silk" is found in several gems but it is particularly common in corundum, spinel and garnet. This is a pattern of very slender, thread like, crystals. When they are fine enough and all arranged in the same direction, you get the impression that you are looking at the gem through a fine layer of silk, or seeing it inside the gem. The greater saturation of the silk, the more brilliance of the gem will suffer.
You are also going to see a number of voids in your gems. Some are bubbles of gas that got trapped inside the crystal as it formed. Besides gas, they may contain a liquid or a crystal. Finding two or three of these in one cavity is a rare treat for the gemologist. It can also be a significant clue as to the identity of the gem.

You may be surprised to see what looks like a fingerprint in a gem. That is called a "healing fracture." At some point in the crystal's history, it got broken. Then the conditions for growth became present and it grew back together, or healed.
Fractures, minerals and voids are the primary type of inclusions you are going to see. However, there are a few specific ones that are of interest. Peridot is known for its "lily pad inclusions." They are a small black spot surrounded by a curved fracture that looks just like a lily pad. This is common and easy to spot.

You may see something similar in a ruby or sapphire. They will sometimes have an opaque inclusion with a fracture around it. They won't be as beautiful and symmetrical as in a peridot. This is an indication that the gem has been heat-treated. When it got hot enough the inclusion burst, creating the fracture around it.

Dematoid garnets are identified by their distinctive "horsetail" inclusions. The name is descriptive and if you ever see one there will be no doubt what you are looking at.