Diamond Flashcards
Etymology of diamond
Greek adamao, I tame/I subdue, 1st c. BC
- also root for adamantine (lustre like a diamond)
- used to denote great hardness
Earliest use of diamond
- first mining occured in India (alluvial deposits)
- Sanskrit vajra, 4th c. BC
Significance of kimberlite
Diamond-bearing kimberlite discovered at Kimberley, South Africa in 1867 resulted in upper and middle classes having access to diamonds
Main diamond producing countries
India (alluvial deposits, prior to 20th c)
Brazil (18th c)
South Africa (1867)
Soviet Union (1950s)
Australia (1980s)
Canada (1990s)
Basic qualities of diamond
- pure carbon
- dense crystalline structure (3.51 g/cm3)
- cubic symmetry
- perfect octahedral cleavage
- Mohs value of 10
- high durability
- high refractive index (2.42)
- great dispersion
- colourless (can show colour due to structural defects, elemental substitution, and lab procedures)
- can fluoresce under UV
- hydrophobic; sticks slightly to grease
- very high thermal conductivity
- low electrical conductance
4Cs of diamonds
Colour
Cut
Clarity
Carat
Thermal conductivity of diamond
Very high due to covalent bonding of the carbon atoms
- 3x higher than gold and silver
Electrical conductivity of diamond
Low
- unusual when paired with high thermal conductance
Crystal structure of diamond
Cubic/isometric crystal system
- each of the crystallographic axes is the same length at 90 degrees to one another (unit cell building blocks are simple cubes)
- very strong covalently bonded carbon atoms & highly symmetrical 3D network
–> hard, durable, dense
Tetrahedron - each carbon atom is bonded to four other carbons in perfec triangular pyramid shape
What is diamond’s weakness?
Cleavage planes intersect to form octahedron
Facets
flat parts on a polished stone
Polymorph
A material of the same composition but with a different crystal structure
Eg.
graphite and diamond both have composition “C”
- in diamond, covalent bonds and cubic crystal system
- in graphite, 2D bonds are strong, but weak Van der Waals bonds between planes (sheets)
How diamonds are grouped
Gemologists and jewellers: 4Cs
Scientists: crystal chemistry variations
Crystal Chemistry Classification
Nitrogen substitution
- Type I –> N > 10 ppm
- Type II –> N < 10 ppm (N-free)
Dispersion of the nitrogen
- Type Ia –> N atoms occur in aggregates within the diamond (~98% of diamonds)
- Type Ib –> N is dispersed iwthin diamond structure
Clustering of the nitrogen
- Type IaA –> N atoms are paired
- Type IaB –> N atoms occur in quads (clusters of 4, often with vacancy at centre)
Boron content
- Type IIa –> B-free (2nd most common type @ <2%)
- Type IIb –> minute (<10 ppm) amounts of B (very rare)
Colouring of each type (crystal chemistry classification)
Yellow hue (absorption of blue light)
- Type Ia (appreciable N, clustered)
White (lack of impurities; no absorption of light across visible spectrum)
- Type IIa (no appreciable N or B substitutions)
Blue to grey hue (most light except blue is absorbed)
- Type IIb (minute B, no appreciable N)
Cause of colour - colourless
No impurities
Cause of colour - Blue to grey
Boron impurities
Cause of colour - yellow to orange, subdued to intense, or almost colourless
Nitrogen impurities
Cause of colour - pink, purple, red, cognac
Deformation of crystal structure (hypothesized)
Cause of colour - green
Natural irradiation (exposure to radiation)
Cause of colour - black
Abundant graphite and other opaque inclusions
Chameleon diamond
Thermochromic - change colour upon gentle heating
Possible habits/shapes of diamond
Most common
- Octahedron
Common
- Cube
- Octahedron modified by cube faces, or cube modified by octahedron faces
Uncommon
- Dodecahedral
- Twinned
- Macle (flat tabular form)
- Polycrystalline aggregate
Corrosive modification during transport can round edges of primary crystal growths; multiple growth and corrosion events can lead to highly complex shapes
Common imitations for diamond
Moissanite (higher dispersion, greater refractive indices, not isometric)
Cubic zirconia (lower thermal conductivity)
Glass (lower thermal conductivity)
Strontium titanite (lower thermal conductivity)
Yttrium-aluminum-garnet (lower thermal conductivity)
Common treatments for diamond - colour altering
- HPHT annealing (high pressure, high temperature)
- LPHT annealing (low p/high t)
- Irradiation
HPHT annealing
- most common colour treatment for diamonds; most often removal of brown body colour/removing or enhancing existing yellow
- increase temperature while maintaining very high pressure
- prevent graphitization
- crystal structure altered (combination states of nitrogen impurities –> change between Type Ia and Ib)
- can heal lattice vacancies
Common treatments for diamond - clarity
- Glass filling (lead-bismuthate glass fills in fractures that reach surface of stone)
- Laser-drilling (high powered laser can reach impurities otherwise sealed from surface; followed by acid boiling and glass filling)
- Acid boiling (acid either bleaches or dissolves inclusion out)
Synthetic diamond production methods
- Chemical Vapor Deposition (CVD)
- High Pressure High Temperature growth (HPHT)
Where can diamonds be found?
Primary deposits
- volcanic rocks on the surface (kimberlite, lamproite)
- un-erupted magma that feed volcanoes
Secondary deposits
- include diamonds that have been moved from primary source and concentrated in a new location (rivers, nearshore currents)
What conditions allow for diamond growth (the Diamond Window)?
Keel of Archean craton
- depth > 140 km (high pressure >50,000 atmospheres)
- “cool region” between 950-1400°C (typical temperature at 140 km depth is ~1500°C)
Kimberlite magmas
- generated at base of craton (200-300km depth), ascend very quickly (picking up diamonds in their path; high speed prevents transformation into graphite and chemical alteration), and erupt in special volcanoes, depositing diamonds on surface
Kimberlite volcanoes
- diatreme –> vertical, carrot-shaped body, typically up to 1km across at surface
- very explosive –> magma interacts with groundwater
- magma travels rapidly; as depth and pressure decreases, magma propogates faster and faster
Indicator minerals
Other minerals occurring in diamond-bearing kimberlites
–> presence on surface can indicate nearby kimberlite rocks
- green olivine
- purple pyrope garnet
- chromium-bearing diopside
- chromium-bearing spinel
- ilmenite (iron titanium oxide)
Kimberlite
- the major source of diamonds
- a form of peridotite (intrusive igneous rock; coarse grained, dark-coloured, dense; min. 40% olivine)
- mica-rich
- occurs in pipes (intrusive igneous bodies with vertical sides; <1km diameter)
- found in uplifted centres of continental platforms; formed during Late Cretaceous period (100-65mil years ago)
Lamproite
- similar to kimberlite; hosts diamond (less commonly)
- unlike kimberlites, can exist in areas outside of Archean cratons
Why is diamond rare?
- formational environment is well below surface; only special geological conditions allow transport upwards
- not as rare as De Beers wants you to think
–> 140 million carats mined annually
Archaen craton
Archaen –> older than 2.5 billion years (to 4 billion); the earlier eon of the two formal divisions of Precambrian time (about 4.6 billion to 541 million years ago) and the period when life first formed on Earth.
Craton –> a large, coherent domain of Earth’s continental crust that has attained and maintained long-term stability. Reworked crust only becomes a craton once the cumulative tectonic, magmatic, and metamorphic reprocessing has self-organized the crust and underlying lithosphere into a stable density, compositional, and thermal profile
4 Cs
System introduced by GIA in 1940’s
Cut
Clarity
Colour
Carat
Bonus: 5th C
Country of origin (proposed)
Cut
The external anatomy of a gemstone; the quality of the facets that define the gemstone’s proportions
ie., the way facets are cut (not the shape)
Grading assessment
- quality of facets and their polish
- physical proportions of the stone
- girdle diameter, angles for crown and pavilion
- excellent/ideal (good symmetry of facets; good length to width ratios)- very good - good - fair - poor
Anatomy of a round brilliant-cut diamond
Facet names of a round brilliant-cut diamond
Clarity
The internal and external imperfections of a stone
Often present in rough form; diamond cutters will often sacrifice carat weight to improve clarity
Six-tiered grading system
I (included) to FL (flawless)
Flaws:
- solid mineral inclusions (most common)
- fluid-filled inclusions
- clouds
- feathers
- scratches
- abrasions
- burns
GIA clarity grading subdivisions
Stones are examined under 10X magnification
Flawless (FL): no blemishes or inclusions
Internally flawless (IF): no inclusions, insignificant blemishes
Very very slightly included (VVS1 > VVS2): contains minor inclusions, very difficult to locate
Very slightly included (VS1 > VS2): contains minor inclusions (difficult –> easy to see)
Slightly included (SI1 > SI2): contains noticible inclusions
Included (I1 > I2 > I3): contains obvious inclusions that may affect potential durability, transparency, and/or brilliance
Colour
- almost all diamonds have yellow undertone
Colour scale:
- D (colourless) to Z (deep yellow, considered undesirable)
- beyond Z: fancy or fancy intense (saturated enough to be unusual, more desirable)
Carat
Total weight of the stone
- one carat is equal to 0.2 grams (or 100 points)
- 5 carats equal 1 gram (or 500 points)
Shapes
- designed to maximize brilliance and fire, or intensify colours
Kimberly Process (Certification Scheme)
Mandate: “to address the development, implementation, and oversight of a tracking system for the export and import of rough diamonds to prevent the exploitation of diamonds for illicit purposes such as war and inhumane acts”
81 countries are involved
- 99.8% of global diamond production
