Added value
As mentioned, some inclusions are wanted in gems, sometimes even pursued, because they improve the price and quality. For instance, quartz with rutile inclusions (normally, rutile inclusions are long needle-like and golden), are much more valuable than simple, colourless quartz. Like golden rutile, black tourmaline and red hematite inclusions give colour and life to a stone, and an even better price.
A green demantoid garnet from Russia with a perfect horsetail inclusion is desirable because of its beauty and the rarity of such a formation—the most beautiful specimens have a horsetail right in the middle of the stone, symmetrical, with long and flowing golden threads going through it.
From a perfect small spinel octahedron, to hundreds of needle inclusions parallel to each other in a sapphire that create an asterism or star sapphire, it’s possible to find rare, unique perfect crystal formations in gemstones.
Risky business
Gemstone setters know how a well-placed fissure can be risky business. Some smaller ones are usually no problem, but they are a less desirable trait found in a gem. If the fissure is not easily visible in the gem because it was oiled (most emeralds on the market are oiled to hide some flaws), it can be detrimental to the structure of the stone.
Diagnostic inclusions
Some inclusions don’t play a role in the valuation process nor the structural integrity of the stone, but are practical in discovering its identity and/or if it’s natural or not. For example, a centipede inclusion in a moonstone is considered a ‘diagnostic inclusions,’ as it proves the stone, without a doubt, is moonstone. Other diagnostic inclusions would be an orderly pattern of small octahedral crystals for spinels, or what is called a ‘lily pad’ inclusion in a peridot—it is a chromite crystal with a decrepitating halo around it.
These are all natural inclusions found in gemstones. There are also inclusions found in synthetic gems and imitation gems like glass or plastic.
