Home » Diamonds » Do SI Diamonds Pass a Diamond Tester?

Do SI Diamonds Pass a Diamond Tester?

We’re reader-supported. Posts may contain links that provide a commission at no cost to you. Learn how we make money.

Do SI Diamonds Pass a Diamond Tester

If you’re searching for a diamond, you’re likely concerned about its authenticity. After all, it’s a significant purchase in both its financial cost and the emotion behind it.

There’s a difference in value between a real and fake diamond, so if you unknowingly buy a simulant, you will have overpaid for the gem.

Slightly included (SI) diamonds are a popular choice for many types of jewelry, especially engagement rings.

The SI designation isn’t referring to whether the diamond is real or fake. It indicates the extent of the diamond’s inclusions and how it affects its appearance, brilliance, and durability.

I’ll walk you through the details of when SI diamonds pass a diamond tester, how those tests work, ways to ensure a diamond is real, and if an SI diamond is right for you.

What is an SI Diamond?

An SI diamond ranks toward the bottom of the clarity developed by the Gemological Institute of America (GIA) because its inclusions are sometimes visible to the naked eye.

The GIA scale ranges from flawless to I3, which each step down means the blemishes are more impactful on its overall performance.

GIA Clarity Scale

These inclusions take the form of:

  • Needles: Long, thin lines within the diamond’s inner surface that are often white or transparent
  • Feathers: Small fractures or cracks that have the potential to cause structural issues
  • Cavities: Craters in the diamond surface where a portion was dislodged during polishing
  • Twinning wisps: A combination of pinpoints, clouds and crystals

An SI diamond has one or more of these inclusions, among several other types.

The SI grade on the clarity scale is broken down into two categories: SI1 and SI2

An SI2 diamond has more impactful inclusions, but it’s still a subjective judgment by the gemologist.

Check out this SI2 diamond.

SI2 Diamond

The GIA report shows six types of inclusion scattered across its table and pavilion. They’re easily visible in the high-resolution photo, which means you’d likely notice them when viewed in a normal setting.

Now compare it to this SI1 diamond.

SI1 Diamond

There are needles and crystals spread across its facets, but they aren’t as apparent in the image. Because it’s a round diamond with an excellent cut grade, it has a higher likelihood of appearing eye-clean.

Fake diamonds like cubic zirconia and moissanite can have imperfections as well, so you can’t determine whether a diamond is real solely based on an SI clarity grade.

The grade isn’t referring to the physical and chemical properties of the gem. Instead, it identifies the clarity characteristics and their impact.

How do Tests on SI Diamonds Work?

Diamond testers are used to detect fraudulent diamonds so the buyer is protected.

Most are portable devices that feature a scale of one to eight, with a color-coding system that flashes red or green to indicate whether a diamond is real.

To learn whether an SI diamond passes a diamond tester, place the diamond near the tip of the device. It’ll detect how quickly heat or electricity moves throughout the diamond.

Diamond Tester

This is an effective method because heat and electricity flow through a diamond differently than throughout other gems like cubic zirconia or white sapphire.

One exception is moissanite. It conducts heat similar to real SI diamonds. Devices that test for electricity are more reliable because moissanite conducts heat in a similar way as a diamond.

Professional jewelers use more complex technology to ensure accuracy. While diamond testers are generally reliable, it’s best not to rely on the basic tool when making decisions about a gem that could be worth thousands of dollars.

You should note that these tests cannot determine whether an SI diamond is natural or lab-grown. Similar to the cost of a real diamond compared to a simulant, lab-grown diamonds carry far less value.

How Do You Ensure It’s Real?

There are several ways to ensure an SI diamond is real, as opposed to a simulant or one grown in a lab.

The surest way to select a diamond that’s been certified by a reliable organization like the GIA or American Gem Society.

Reputable jewelers always offer the certificate with their diamonds, and it should match the serial number inscribed on the girdle.

As an example, here’s an SI diamond graded by the GIA.

SI Diamond

You can see at the top of the report that its serial number is “7418597228,” and it can be verified on the GIA’s website.

I inputted the serial number on the site, and the GIA returns its results, which you can see below.

GIA Report for SI Diamond

At the top, the report confirms it’s a natural diamond.

If you’re considering an SI diamond that isn’t certified, there are two steps to learning if it’s real.

The first is to use a diamond tester. 

If it passes, but you aren’t sure whether it’s natural or lab-grown, you’ll have to submit it to a gemologist for their review. 

Even a trained professional often can’t tell the difference without specialized equipment.

The image below provides an example.

Lab-Created Diamond and Natural Diamond

The diamond on the left is lab-grown, and on the right is a natural one. They appear identical.

That’s why I always recommend choosing a certified diamond from a trusted jeweler. You’ll avoid the risk of buying a simulant or man-made diamond and can instead have confidence in its quality.

Should You Buy an SI Diamond?

SI diamonds are considered a strong value because they can appear identical to ones with higher clarity grades without magnification. 

You’ll avoid the premiums for VS or VVS clarity and still have a diamond that looks flawless. 

An SI1 diamond is more likely to meet that criteria than SI2, but it depends on the specific inclusions and its carat weight. Always view it in person or through high-resolution images.

After you learn whether your SI diamond is real, either through a diamond tester or from certification by a credible institution, you can pair it with the right setting and land on the perfect piece of jewelry for you.

Jacob Clarke

Jacob Clarke

Jacob Clarke is the founder of TeachJewelry.com.

He earned an Applied Jewelry Professional Diploma from the Gemological Institute of America (GIA) and now brings you essential information about diamonds, settings, and more.

Jacob has consulted with leading jewelry brands, and his work has been cited in Clean Origin, Diamond Nexus and industry publications.

He's also a member of the International Gem Society.

He enjoys discussing jewelry with readers, so contact him with any questions at jacob.clarke@teachjewelry.com.

Learn More About Jacob