Crystals, Needles, Knots, and Metallic Inclusions

Chapters 41 and 42 asked what an inclusion is geologically. This chapter shifts perspective and asks how solid inclusions are described during gemological examination.

That is an important distinction. A clarity term describes what is seen and how the feature affects clarity assessment. It need not be a mineral diagnosis or a test of natural origin.

Four Separate Questions

For every solid inclusion, distinguish:

1. observational name—crystal, needle, knot, or another clarity characteristic;
2. mineral identity—if analytically determined;
3. genetic significance—what the inclusion reveals about growth or origin;
4. gemological effect—visibility, transparency, position, and any relationship to the surface.

Conflating these four levels is the source of many erroneous conclusions.

Crystal

GIA’s term crystal describes a mineral crystal contained within a diamond. In practical grading, it is a morphological category, not an assertion that a mineral species has been determined.

A crystal may be:

• colorless or colored;
• transparent, dark, or metallically reflective;
• well formed or irregularly resorbed;
• isolated or part of a group;
• fully enclosed or connected to the surface;
• accompanied by a pressure halo or not.

For clarity, its relief—the optical contrast with the diamond—is often more important than the inclusion’s actual bodycolor.

Color and Relief

A dark crystal draws the eye more readily than a similar colorless inclusion. An inclusion, however, may also appear dark because of reflections from its surroundings, total internal reflection, or multiple images created through facets.

A single “black spot” should therefore not automatically be identified as graphite or metal. Rotate the stone, change the illumination and focus, and distinguish the actual inclusion from its reflections.

Needle

A needle is a thin, elongated crystal inclusion that resembles a tiny needle or rod at standard magnification.

The term does not determine mineral composition. A needle may occur alone, in a group, or as part of a more complex growth structure. The orientation of multiple needles may be geologically or crystallographically interesting, but by itself it does not prove a specific growth mechanism.

A needle should be distinguished from:

• a pinpoint, which is extremely small and dot-like;
• a cloud, which consists of a population of numerous tiny inclusions;
• a polish line, which is a surface finishing mark;
• a narrow reflection from a larger crystal or feather.

[VISUAL 43.1: Crystal, needle, pinpoint, and cloud—the same examination scale, four different clarity terms]

[VISUAL 43.2: Knot, natural, indented natural, and cavity—cross section of their relationships to the polished surface]

Knot

A knot is a diamond crystal that reaches the polished surface after cutting. It therefore differs from a fully enclosed crystal inclusion.

Because it is a diamond crystal within a diamond, with a different crystallographic orientation, the knot’s surface may respond differently to polishing. Under certain illumination, a boundary, a change in luster, or relief may be visible.

A knot should be distinguished from:

• natural—a remnant of the rough diamond’s original surface;
• indented natural—a portion of the original surface that extends beneath the imagined polished outline;
• cavity—an actual opening or depression caused by missing material.

Crystal Dropout and Cavity

A mineral inclusion that reaches the surface may fall out partly or completely during polishing. The result may be a cavity.

This differs from a knot: in a knot, the inclusion is a diamond crystal that remains at the surface; in a dropout, material has been lost and an opening remains.

The final shape of the cavity alone may not reliably reconstruct what was previously present at that location.

Pressure Halo

A mineral inclusion may have elastic and thermal properties different from those of diamond. Changes in pressure and temperature during ascent may create strain around the inclusion and a series of local fractures—a pressure halo.

It is important to distinguish:

• the primary solid inclusion;
• the fractures surrounding it;
• reflections from those fractures;
• their combined effect on clarity.

A small inclusion may therefore appear much larger than it actually is.

Graphite, Sulfides, and Other Dark Inclusions

Natural diamonds may contain graphite, sulfide, oxide, and other dark mineral phases. Some are scientifically very valuable, but several causes can produce a similar black or metallic appearance under a gemological microscope.

Mineral identity therefore cannot be determined solely from:

• black color;
• metallic luster;
• irregular shape;
• a magnetic response from the entire stone.

Raman spectroscopy or other targeted methods provide much stronger evidence.

Metallic Inclusions and Two Entirely Different Contexts

Metallic phases are especially sensitive because they occur in at least two important contexts.

In some natural superdeep diamonds, particularly CLIPPIR diamonds, inclusions related to Fe–Ni–C–S metallic liquid in the deep mantle have been documented.

Conversely, HPHT laboratory-grown diamonds may contain metallic remnants associated with the growth medium.

A “metallic inclusion” by itself therefore does not reveal whether a diamond is natural or laboratory-grown. Morphology, composition, growth patterns, spectroscopy, and the complete laboratory context are decisive.

Magnetism and Conductivity: Screening, Not a Verdict

Some metallic inclusions may produce a measurable magnetic response. This can be a useful screening clue, but the result depends on the size, quantity, and composition of the metallic phase, the diamond’s mass, orientation, and the test’s sensitivity.

Likewise, electrical conductivity of the entire diamond may originate in the crystal lattice—for example, in boron-bearing diamonds—rather than in an inclusion.

Neither magnetism nor conductivity should be turned into a stand-alone diagnosis of origin.

Reflections Multiply the Observation, Not the Inclusions

Facets can create multiple reflections of a single crystal or needle. A stone may therefore appear to contain several identical inclusions.

A practical check includes:

• changing the focus;
• rotating the stone;
• observing it through the crown and pavilion;
• tracking how the “copies” behave as the stone moves.

The repeated images arise from the stone’s optics; they do not indicate multiple actual inclusions.

[VISUAL 43.3: One crystal and its facet reflections—the actual inclusion versus optical “copies”]

[VISUAL 43.4: Metallic inclusion—a natural deep-Earth context versus an HPHT growth-remnant context, without an automatic diagnosis]

What Determines the Effect on Clarity

As with other characteristics, the important factors are:

• size;
• number;
• position;
• nature;
• relief.

A small dark crystal beneath the table may be more prominent than a larger colorless one near the girdle. A single needle may be insignificant, while a large group may become the dominant characteristic.

This is not a new clarity methodology—it was covered in Chapter 28—but an application to these specific morphologies.

When Analytical Confirmation Is Necessary

The mineral species need not be determined if the question is simply “which clarity characteristic affects the grade?” Analytical confirmation becomes important when it is claimed:

• that the inclusion is a particular mineral;
• that it belongs to a particular paragenesis;
• that it proves natural or laboratory-grown origin;
• that it has geological or geochronological value.

Methods from Chapter 41 and the instrumental framework in the later Part IX then become relevant.

Practical Examination Protocol

1. clean the stone;
2. examine it at standard magnification from multiple directions;
3. distinguish crystals, needles, knots, and their reflections;
4. determine whether the feature is enclosed or reaches the surface;
5. record any pressure halo and secondary fractures;
6. do not name a mineral based only on color or luster;
7. treat a magnetic response only as a supporting clue;
8. seek analytical confirmation only when the conclusion requires it;
9. never determine a diamond’s origin from a single microscopic feature.

Chapter Summary

• Crystal, needle, and knot are clarity terms, not complete mineral diagnoses.
• Color and relief influence an inclusion’s visibility but do not determine its identity.
• A needle is a thin, elongated crystal inclusion; pinpoint and cloud belong to other observational categories.
• A knot is a diamond crystal that reaches the polished surface.
• Crystal dropout may leave a cavity, but the final opening may not reveal its exact history.
• A pressure halo may optically magnify the effect of a small inclusion.
• A black or metallically reflective inclusion is not automatically graphite or metal.
• Metallic phases occur in both natural deep-Earth and HPHT laboratory-grown contexts.
• Magnetism and conductivity are supporting clues, not stand-alone tests of origin.
• Facet reflections may create the appearance of multiple inclusions.
• The effect on clarity depends on size, number, position, nature, and relief.
• Mineralogical confirmation is necessary when an observational term is used to draw a geological or identification conclusion.