Diamond Properties and Grading Flashcards

A complete set of 60 vocabulary flashcards covering diamond properties, grading, treatments, synthetics, and localities based on the Gem-A Diploma study guide.

Adamantine lustre

The exceptionally high-brilliance surface lustre characteristic of diamond, which is unique among natural colourless gemstones.

Dispersion

The splitting of white light into spectral colours, also known as 'fire'; for diamond, this value is fixed at $0.044$.

Fire

The play of spectral colours seen in a cut diamond, produced by its extremely high dispersion of $0.044$.

Brilliance

The proportion of white light reflected back to the eye through total internal reflection, which is dependent on cut proportions.

Total Internal Reflection (TIR)

A phenomenon occurring when light hits the back facets of a well-cut diamond at an angle exceeding the critical angle and is entirely reflected back.

Dot test

A test where a well-cut round brilliant diamond is placed table-down over a printed dot; if it is diamond, the dot will NOT be visible due to TIR.

Kimberlite

A volcanic rock type in which diamonds are hosted, named after Kimberley, South Africa.

Lamproite

A volcanic rock type hosting diamonds, with the Argyle mine in Australia being a notable example.

Trigon

Triangular etch pits found on the octahedral faces of rough diamond crystals; they serve as a key recognition feature for natural diamonds.

Macle

A triangular twinned diamond crystal, which appears as a flattened twin of two octahedral halves and shows a re-entrant angle.

Grain lines

Internal structural irregularities visible as lines in cut diamonds that are characteristic of the species.

Feather

A fracture in a diamond that appears feather-like and affects the clarity grade.

Natural

A small area of the original crystal face left on the girdle of a cut diamond, sometimes showing trigons or herringbone markings.

Cape series

A range of colourless to yellowish diamonds (Type Ia) coloured by nitrogen aggregates, showing a characteristic N3 absorption band at $415\,nm$.

Nitrogen (N)

The most common trace element in diamond, responsible for yellow tints in Type Ia and Type Ib stones.

Boron (B)

A rare trace element in Type IIb diamonds that causes a blue colour and makes the gemstone an electrical semiconductor.

Type Ia

The most common natural diamond type, containing nitrogen in aggregates and showing blue fluorescence under LW UV.

Type Ib

A diamond type containing nitrogen as isolated atoms, producing yellow to orange ('canary yellow') colours; common in HPHT synthetics.

Type IIa

Diamonds containing negligible nitrogen; they are the purest, most colourless diamonds and have no sharp visible absorption.

Type IIb

A rare diamond type containing boron, making it blue and an electrical semiconductor; it shows red fluorescence under SW UV.

Plastic deformation

Distortion of the crystal lattice believed to cause brown and possibly pink or red colours in diamonds.

GR1 band

An absorption band at $741\,nm$ caused by vacancies from irradiation, diagnostic for green diamonds.

HPHT (High Pressure High Temperature)

A treatment that can decolourise brown diamonds or produce fancy yellow-green colours; also a manufacture method for synthetics.

Fracture filling

A non-permanent treatment where lead-glass is injected into surface-reaching fractures to improve apparent clarity, creating flash colours.

KM laser treatment

A treatment where a laser heats a dark inclusion to create stress fractures, allowing acid to dissolve the inclusion through a zig-zag channel.

DiamondSure™

A DTC instrument that detects the $415\,nm$ absorption band to confirm a stone is a natural diamond.

DiamondView™

A DTC instrument that uses UV-excited fluorescence to reveal cubo-octahedral growth patterns in synthetic diamonds.

4 Cs

The four quality factors used to grade diamonds: Carat weight, Colour, Clarity, and Cut.

Carat weight

The weight of a diamond where $1\,ct = 0.2\,g$; price increases significantly at 'magic weights' like $1.00\,ct$.

Master stones

Pre-graded colour comparison diamonds (minimum $0.50\,ct$) used to colour grade other stones.

Round brilliant cut

The most popular diamond cut, consisting of $57$ facets, or $58$ if a culet is present.

Scintillation

The lively sparks produced by light intersecting with facets as the stone or observer moves.

Cubo-octahedral habit

The distinctive internal growth form combining an octahedron and a cube found in synthetic HPHT diamonds.

Synthetic moissanite

A dangerous diamond simulant with an RI of $2.65$ to $2.69$ and strong double refraction; it is both a thermal and electrical conductor.

Thermal conductivity probe

A portable instrument used to detect diamond's high thermal conductance; it also reacts positively to synthetic moissanite.

Conflict diamonds

Diamonds mined in war zones and sold to finance armed conflict; international procedures prevent their export.

Cubic system

The crystal system of diamond, which makes the stone isotropic (singly refractive).

Specific Gravity (SG)

The measurement of diamond's density, which is a constant value of $3.52$.

Refractive Index (RI)

The measurement of light bending in diamond, which is $2.42$, the highest of any natural gemstone.

Hardness

Diamond's resistance to scratching; it is the hardest known natural substance, rated at $10$ on the Mohs scale.

Cleavage

Diamond's tendency to split in four directions along perfect octahedral planes, making it vulnerable to sharp knocks.

Toughness

The resistance to breakage, which is only 'Good' for diamond because of its perfect cleavage.

Dodecahedron

A 12-faced rhombic crystal habit common in diamonds, often showing curved faces due to dissolution.

Indicator minerals

Minerals such as garnet and spinel found in kimberlite that suggest diamond exploration may be successful nearby.

Alluvial deposits

Diamonds found in river and beach gravel deposits after being transported from volcanic pipes.

Argyle mine

A famous locality in Australia known as the major source for pink, red, champagne, and brown diamonds.

Olivine

A common mineral inclusion found within natural diamonds.

Clouds

Concentrations of very small inclusions that create a hazy appearance within a diamond.

Laser drill holes

Fine channels reaching from the crown to dark inclusions, intended to allow acid to bleach the inclusion.

Bruting

The stage of diamond manufacture where the girdle of the stone is rounded.

Symmetry

A cut grading component that evaluates how symmetrical a stone appears, such as the position of the table and culet.

Polish

The quality of the surface finish on a diamond, which can be affected by burn marks or polishing lines.

Anomalous extinction

A diagnostic reaction sometimes seen in diamonds when viewed under a polariscope.

D colour grade

The highest GIA colour grade, indicating a stone that is colourless both table-up and table-down.

Flawless (FL)

The highest GIA clarity grade, where no inclusions or blemishes are visible under $10\times$ magnification.

I3 (P3) grade

The lowest clarity grade, indicating obvious inclusions that seriously affect beauty or durability.

Metallic flux

Diagnostic inclusions found in synthetic HPHT diamonds that may cause them to be attracted to rare earth magnets.

Belt process

One of the two HPHT methods used to produce commercial gem-quality synthetic diamonds.

BARS method

A split sphere/split half method used in the laboratory synthesis of HPHT diamonds.

Pavilion depth

The most critical proportional angle in a cut diamond; light leaks out if it is too deep or too shallow.