REMARKABLE FACTS:
All diamonds are at least 990,000,000 years old . Many are
3,200,000,000 years old (3.2 billion years)!!!
HOW DO WE KNOW THIS ?
Age: from Carbon dating ??? NO!!...... C-dating only works for very
young carbon. You need to use other radioactive decay schemes (e.g.,
uranium-lead) to date inclusions in diamonds. Inclusions used for dating
are around 100 microns in diameter (0.1 mm).
Diamonds are formed deep within the Earth: between 100 km
and 200 km below the surface.
Diamonds form under remarkable conditions!
The temperatures are about 900 - 1300 C in the part of the
Earth's mantle where diamonds form
The pressure is between 45 - 60 kilobars (kB)
50 kB = 150 km = 90 miles below the surface
60 kB = 200 km = 120 miles below the surface
Diamonds are carried to the surface by volcanic eruptions.
The volcanic magma conduit is known as a kimberlite pipe or diamond
pipe. We find diamonds as inclusions in the (rather ordinary looking)
volcanic rock known as kimberlite.
NOTE: The kimberlite magmas that carry diamonds to the surface are
often much younger than the diamonds they transport (the kimberlite
magma simply acts as a conveyer belt!).
Diamond is made of carbon (C), yet the stable form
(polymorph) of carbon at the Earth's surface is graphite.
To ensure they are not converted to graphite, diamonds must be
transported extremely rapidly to the Earth's surface.
It is probable that kimberlite lavas carrying diamonds erupt at between
10 and 30 km/hour (Eggler, 1989). Within the last few kilometers, the
eruption velocity probably increases to several hundred km/hr
(supersonic!).
HOW RARE ARE DIAMONDS?
How many grams do you need to mine to get 5 grams of diamonds?
(5g/1000 kg) @ 1000 g/kg = 5 g /1,000,000 g!
BUT only 20 % are gem quality (80 % of these are sold in a 'managed
selling environment') and the remainder are used for industrial purposes
(this material is known as 'bort' or 'carbonado' (carbonado is finer)).
CHARACTERISTICS OF DIAMOND:
Hardness = 10
Crystal System = cubic
This is what crystals look like before they are faceted: note their natural
octahedral shape!
Uncut diamonds are also found in cubic forms
Diamond has four good cleavages, thus diamonds tend to cleave on
impact
Other diagnostic properties.
Famous diamonds
This is just for fun -- not required information!
1.Dresden
2.Hope ...the real thing!
3.Cullinan (Before)
After
4.Sancy
5.Tiffany
6.Kohinor
a-section
7.Shah
8.Nassau
9.Florentine
10.The Great Mogul
11.Orloff
12.Stern
13.Regent
VALUE
The 4 "C" words are used to summarize the value determining
factors:
1.COLOR
2.CLARITY
3.CUT
4.CARAT WEIGHT
The above links are all to remote sites. The required basic information
describing what is meant by these terms is provided below.
Some further comments on the '4 C's from another remote source!
(1) COLOR: is determined by 'grading' visual comparison with 'knowns'
or by instrumental means.
consider the amount of yellow color (yellowish color decreases
the value of a "colorless" stone). In order of increasing yellow
content:
blueish-white -> white -> silver -> yellow
'Fancy', or strongly colored stones have their own appeal and
special value.
Colored diamonds may beyellow, green or brown, green or
shades of pink
Larger pink diamonds are quite rare and currently very !!
expensive.
Natural blue diamonds contain the element boron (B), and this
changes the conductivity of the diamonds. Natural yellow
diamonds contain the element nitrogen (N).
(2) CLARITY : Clarity is decreased by the presence of blemishes or
flaws, scratches, nicks, 'naturals' (the original surface of an uncut stone).
There are many systems of nomenclature.
Some terms include:
flawless or perfect
imperfect
very slightly imperfect
very very slightly imperfect
F1 VVS1 VVS2 VVS3 VS1 VS2 SI1 SI2 I1 I2 I3
flawless imperfect
other descriptions:
"first pique" inclusions readily recognizable at 10x mag., not significantly
diminishing brilliance
"second pique" larger inclusions, can be seen with naked eye
"third pique" many large inclusions, diminishing brilliance
Examples of clarity-reducing inclusions:
inclusions
cracks
a crack along the pavillion
(3) CUT
Facets are placed so as to maximize the brilliance and fire of a stone.
Remember that in the first lecture we talked about how the proportions
of a faceted gemstone are determined based on the refractive index?
Review the basic concepts:
Refraction is dependent upon the wavelength
Refractive Index (RI) is proportional to wavelength; red RI <
violet RI (dispersion is due to the different amounts different
wavelength are bent)
Fire,which is seen as rainbows and glints of color, is due to
dispersion (a consequence of the placement of faces on the
crown to take advantage of the prism effect).
Review the light path in a correctly cut gem!
The brilliant cut (American cut) is a typical cut chosen for diamonds.
There are many alternative diamond cuts
A poorly cut stone is characterized by poorly chosen proportions (poor
optimization of brilliance and fire or, worse still, leakage of light from the
pavillion). Misplaced facets, extra facets, and problems at facet junctions
are also characteristics that reduce the quality of "cut".
Ranking: VERY GOOD ... GOOD .... MEDIUM ... POOR
MM |
