arrow_back Back Next arrow_forward
diamond Inclusion

Crystal

GIA Crystal
crystals as seen in a diamond.
An amber colored crystal - stone can usually be identified.
Crystal plot (red rectangle highlights the crystal plot on the image and is not on the actual plot)

GIA Gemology Guide: Crystal Inclusions in Diamonds

For a GIA student or professional jewelry appraiser, mastering the identification and evaluation of clarity characteristics is essential. A crystal is one of the most fundamental and fascinating internal characteristics of a diamond, offering a literal window into the gem's formation deep within the Earth.

1. What is a Crystal?

A crystal is defined as a mineral that is contained within a diamond. It can be a completely different mineral—such as garnet, diopside, spinel, or olivine (peridot)—or it can be another, smaller diamond crystal that was enveloped by the host diamond as it grew.

How do they form?

Crystals are entirely natural and form under the same extreme heat and pressure as the host diamond itself. They occur when:

  • A preexisting mineral in the earth's mantle is surrounded and trapped by the growing diamond lattice.
  • Another mineral forms at the exact same time as the diamond and is encapsulated during the crystallization process.

Note: In the trade, dark or black crystals are often colloquially referred to as "carbon spots." However, gemologically, these are usually just dark inclusions like graphite or sulfides, and GIA simply classifies them as crystals.

2. How to Plot a Crystal (GIA Standards)

Because a crystal is completely enclosed inside the diamond's crystal lattice, it is classified purely as an internal characteristic (inclusion) and is plotted using red ink.

Plotting Steps:

  • Locate the Feature: Using a microscope with darkfield illumination or a 10x loupe, locate the crystal. Examine it from different angles to determine its shape, depth, and whether it reflects off adjacent facets (a phenomenon known as a reflection).
  • Draw the Outline: A crystal is plotted by drawing a solid red outline that mimics the shape of the crystal as closely as possible.
  • No Fill: Even if the crystal is dark or opaque, do not color it in. It is simply plotted as a red outline.
  • Do Not Plot Reflections: If the crystal reflects around the interior of the diamond, remember that GIA standards dictate that reflections are never plotted. You must locate and plot the actual inclusion only once in its true physical location. However, keep in mind that the visual multiplication caused by reflections is factored into the diamond's final clarity grade.
  • Key to Symbols: "Crystal" is listed in the Key to Symbols. If the crystal is the most prominent inclusion dictating the clarity grade, it will be listed first.

3. Crystal vs. Similar Characteristics

It is important to differentiate a fully enclosed crystal from other internal or surface-reaching inclusions:

  • Crystal vs. Pinpoint: A pinpoint is actually a microscopic crystal. The distinction lies in size. If the inclusion is so small that it only appears as a tiny dot at 10x magnification without a resolvable shape, it is a pinpoint (plotted as a single red dot). If you can resolve its shape at 10x, it is a crystal.
  • Crystal vs. Knot: A knot is a white or transparent diamond crystal that extends to the surface after polishing. Because it breaches the surface, a knot is plotted with both green and red ink. A standard crystal is fully enclosed and plotted only in red.
  • Crystal vs. Cloud: A cloud is a dense, hazy grouping of microscopic pinpoints or crystals. A crystal is a distinct, individual, and resolvable mineral.

4. Evaluation and Appraisal Considerations

For appraisers and gemologists, evaluating a crystal involves looking at its size, location, and most importantly, its contrast against the host diamond.

  • Impact on Clarity Grade: The effect on the clarity grade depends on the "relief" or visibility of the crystal. A transparent or "white" crystal located near the edge of the stone might keep the diamond in the VS range. However, a highly visible black crystal situated directly under the table facet will easily drop the grade into the SI or I (Included) categories.
  • Durability Risks: Generally, a crystal poses little to no durability risk because it is securely encased within the hardest material on earth. However, if a very large crystal is accompanied by internal stress fractures (feathers) or sits dangerously close to a sharp corner (like the point of a princess cut), it could present a minor risk during setting.
  • Appraisal Value: The color and location of a crystal heavily influence a diamond's market appeal and value. Two diamonds with the exact same SI1 grade can have vastly different visual appeal. An SI1 with a transparent, barely noticeable crystal on the pavilion is much more desirable—and will command a higher price—than an SI1 graded due to a high-contrast, black crystal right in the center of the table. "Eye-clean" SI diamonds trade at a premium over non-eye-clean ones.

Summary for GIA Students

  • Definition: A mineral crystal completely contained within the host diamond.
  • Plotting: A solid red outline mimicking the shape of the inclusion (reflections are not plotted).
  • Key Factors: Pay close attention to relief/color (white vs. black) and location (table vs. girdle). Ensure you distinguish a crystal from a pinpoint (based on size) and a knot (based on whether it reaches the surface).


Previous Characteristic Next Characteristic