Diamond Clarity- What is a "Feather"?
When you shop for diamonds, you will notice that the clarity has a great impact on the price.
A small percentage of diamonds are graded Internally Flawless ( under a 10power magnification device) by GIA. The rest have some sort of flaw.
There are many different types of "imperfections" present in diamonds. A "feather" is an area in the diamond which blocks the light. Diamonds have grain and this type of imperfection usually runs along an internal grain line of the mineral.
The photo below is an extreme example. Like a bird's feathers, there can be many different types of feathers in diamonds.
Many feathers are tiny, and translucent. Many eye clean ,desirable diamonds may contain a feather. In the example below- a Vivid Yellow I1 Oval Diamond, the feather is clearly visible from the back
Although visible from the front too, it's much less noticable
Here's a feather in a round diamond
Severity
A feather may be so very tiny that it will be graded VVS1 by GIA- or it may look like the diamond with the huge opaque white boogers.
In the photo of the round diamond below, the feathers are completly opaque, and HUGE. Such a diamond is only for folks who don't mind seeing the imperfection- and there are quite a few who feel this way. I'd grade the round diamond I2 ( it does not have a GIA report)
In terms of the clarity grade, GIA bases the grade on the presence of imperfection- NOT- the visual impact.
In other words: A teeny black pinpoint in the center of a diamond might be called VS2. Compare that with a diamond which has a rather large SI2 sized feather- but the feather is located in such a spot that it's totally invisible unless you turn the diamond upside-down. Because many feathers are actually very "soft" imperfections, they can be a blessing.
Say you were considering a two carat F color diamonds: an F SI1 with a small feather looked good, versus an F/VS1 with a tiny pinpoint. If you chose the SI1 you'd save about nine thou$and dollars
Feathers, and Fancy Shapes
When considering the effect of a feather ( or any imperfection) on a diamond, it's shape and cutting style will often play a role.
For example, many emerald cuts with feather imperfections still manage to be eye clean- although many Emerald cuts as clean as VS2 may have an imperfection which is visible to the naked eye. The same feather which might be apparent at a glance in a step cut, like and an Emerald Cut 
, might not be visible to the naked eye on a brilliant faceted diamond, such as a Radiant cut 
.
Is a feather dangerous?
Other than the visual aspect, many buyers want to know if the feather will affect the durability of the diamond.
The answer is: Rarely.
If you're concerned of a feather impacting durability, or desirability, here's what to look for:
- A feather which breaks the surface of the diamond. I am hesitant to buy if the feather breaks the surface on an area that might pose a danger when setting- such as on the corner. This is something to look for in stones with corners, and points- such as : Princess Cuts, Pear Shapes, Marquises.
- If the feather breaks the surface on the table, or below the girdle, on the bottom of the diamond it's rarely a durability issue- although it may be a desirability issue
- Watch out for feathers which span a delicate area. The "booger diamond" posted above is an example of the type of feather which will give a setter pause. It can be set, but a hammer hit in the wrong direction might be catastrophic, causing a piece to fall off, or even shatter the stone.
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