For most facet-quality transparent gemstones, the general rule of thumb is that eye-clean gems rank as more valuable than those with inclusions. Of course there are exceptions to this rule, such as phenomenal stones that gain their visual phenomena specifically from inclusions (i.e. cat’s eye chrysoberyl). Other exceptions could be inclusions that heighten the market appeal of a stone- a good example of this would be golden rutile in colorless quartz.
For the majority though of all gems, people trade them according to a hierarchy of clarity grades. What some people get confused by, is the similarity or difference of grading factors between different gem species. For diamonds, most dealers make use of a scale that ranges from “F” or flawless to “IF” or internally flawless to “VVS” or very very slightly included. Take note though that while there are some gem dealers who describe other stones (like ruby or sapphire) with similar terms as these, the definition of such terms is inherently different.
In today’s market, colorless to near-colorless diamonds tend to have very few or very little inclusions compared to most other gem species. In contrast to this, there are some gem varieties like green beryl or emerald that tend to come with a large variety of inclusions (even in top quality tiers).
Understand that most gemologists follow a general expectation system when it comes to grading colored stone clarity. There exists three clarity grading types and each variety or species falls under one type. Type 1 denotes that facet-quality gems or rough can readily be seen or acquired in eye-clean grades. Topaz, and Aquamarine are some gems that belong to this type. Some gem-cutters even make use of type 1 gemstones to carve miniature sculptures or free-form cuts that play with the their optical purity.
Type 2 stones are simply a notch lower than those that belong to the previous category. These stones can come in eye-clean qualities, but specimens are very rarely seen and often belong to top market batches, commanding a premium due to their scarcity. Rubies and sapphires belong to this category.
Type 3 stones generally almost always have inclusions present. Even the most expensive examples will often show inclusions to some-degree. Emeralds and watermelon tourmaline both belong to this type category. If you’ll examine the beautiful emeralds at auction, you’ll notice that most (while generally very transparent), will still have natural mineral inclusions. Many of such are even used for determining the source origin of these stones.
Diamonds are held to a much stricter standard for clarity. Even a lower clarity graded diamond (i.e SI 1) may appear as clear as an eye-clean stone from another species like aquamarine.
The most minute or tiny inclusions already have a large impact on diamond prices. In contrast, the value-effect of inclusions matters lesser as you go lower on the clarity type scale, with the evaluation of type 3 stones generally putting less gravity on the presence of internal debree or other clarity characteristics.
