“Clarity enhanced diamonds” are confusing. Does "clarity enhanced" mean the diamonds are high or low quality? If you think about it, diamonds that already have high clarity don't need any enhancement. Diamonds that have a need to improve diamond clarity are put through one of a few treatments in order to amend flaws, blemishes, or inclusions.
External Laser Drilling
An otherwise beautiful gemstone may suffer an unsightly inclusion. The most common are carbon inclusions, or colored crystals that become structurally embedded in the diamond. Drilling sounds like it would leave a considerable mark, but the drilling happens at so small a scale, the only sign it was ever done is a microscopic tunnel. This tunnel rises from the inclusion to the diamond's surface.
This is done to improve diamond clarity on diamonds that are otherwise unsaleable. Most of these will never see an exceptional cut because these diamonds simply aren't in high demand. An untreated diamond would usually be a wiser purchase.
Internal Laser Drilling
This is often used as a deceptive practice. Essentially, internal laser drilling won't leave an obviously artificial tunnel. Instead, the drilling will be made to resemble a natural flaw. This way, these diamonds are sold as having natural, feather-like flaw patterns that are in no way natural.
They're often bleached as well. This reduces the apparent visibility of darker inclusions, as well as reducing the obvious artificiality of the drill work. This is a key practice used by scammers who are trying to grade bump their diamonds. GIA and AGS certified diamonds have been tested for this method, so trust the certification process. If a supplier fails to offer certification, be wary.
The final technique used in clarity enhanced diamonds is fracture filling. This involves inserting filler material into the diamond to amend scratches and surface cracks. Glass and other liquid filler materials that share similar refractive properties with diamonds are used.
This has been used by some companies (such as Yehuda) to supply large diamonds at affordable prices. They're transparent about this process. Some customers are content with the trade-off: they get a larger diamond with the knowledge that it's not entirely diamond, that there is some filler material in there. Other customers wouldn't touch a diamond like this.
There's no such thing as a perfect filler at this point – it will always be obvious to an expert if a diamond has used fracture filling. Since fillings degrade after a while, companies that sell fracture filled diamonds usually offer lifetime repairs. The issue is that with each repair, now you've got yet another layer of application that further diminishes the quality, value, and aesthetic of the diamond itself. The process can also never be undone. That diamond will always possess filling material.
Is Clarity Enhanced Worth It?
These methods all offer solutions to improve diamond clarity, but clarity enhanced diamonds are usually not a good investment unless you have a reliable market ready to buy them. There's not a such a market for them in most places, especially with the sheer range of higher clarity diamonds that are already available.