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| Jade |
| GEOBITS
of Jade: |
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Jade
is popular among Chinese. In gemmology, however, only
two minerals are recognised as jade: nephrite and jadeite.
Many jades sold on the market are treated to enhance their
beauty. The tests described here are by no means exclusive
but are some of the common means for differentiating various
kinds of jade.
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Nephrite |
Jadeite
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Mineral Classification
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A member of isomorphous series of
tremolite and actinolite of the amphibole group
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Pyroxene group |
| Chemical Composition
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Hydrous Magnesium-iron Calcium Silicate Ca2(Mg, Fe)5Si8O22(OH)2
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Sodium Aluminium Silicate NaAl(SiO3)2
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| Crystal
System |
Monoclinic |
Monoclinic |
Crystal Habit
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Polycrystalline to cryptocrystalline.
Interlocking masses of microscopic fibrous
crystals |
Polycrystalline to cryptocrystalline.
Granular to fibrous interlocking structure
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Hardness
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6.5 |
Approx. 7
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| Specific
Gravity |
2.8 - 3.1 |
3.30 -
3.36 |
| Cleavage
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Perfect in 2 directions: 124o
and 56o |
Imperfect in 2 directions at 87o
and 93o |
| Fracture
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Uneven to splintery |
Sugary, hacky |
| Colour
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White to yellowish and reddish, dark
green, black, also grey, brown |
White, green, mauve, blue, orange,
brown to black |
| Lustre
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Greasy
to vitreous |
Greasy to vitreous
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| Refractive
Index |
1.62 |
1.65 - 1.67 |
| Locality
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China, New Zealand, Canada, Russia,
USA |
Upper Myanmar, Japan, California,
Russia |
| Remark
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Most precious jade material in ancient
China |
Not known to China before early Ching
Dynasty |
| Jade
Simulants |
Translucent emerald, prehnite, chrysoprase
(chalcedony), bowenite (serpentine), californite
(idocrase), hydrogrossular garnet, aventurine
quartz (Australian jade), dyed quartzite, dyed
marble, glass and plastics |
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Terminology in modern jade trade:
| A
Jade: |
Natural
jadeite without treatment; this class has nothing to
do with quality i.e. colour and transparency. |
| B
Jade: |
Dark
impurities (such as iron oxides) corroded by strong
acids and the material consolidated with resin. |
| C
Jade: |
Dyed
jadeite. |
| B
+ C Jade: |
Jadeite
subjected to the above two treatments. |
| Identification: |
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| 1. |
Tests for jade simulants |
| 1.1
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Jadeite is an aggregate.
The first step of identification is by observation
or using polariscope (a pair of crossed polarizing
filters) to check the stone whether it is
a single crystal or an aggregate. |
| 1.2 |
To
test SG and, or, RI to determine the mineral type. |
| 1.3 |
Plastic is warm in touch.
Glass has bubbles or micro dendritic structure
(devitrification).
Chrysoprase is apple green in colour. Aventurine
quartz is dotted with green spots of fuchsite (green
mica). Serpentine
has a waxy lustre.
Dyed quartzite and marble are like C Jade
where green colour is concentrated at grain edges
and cracks; the latter is effervescent in dilute
HCl. |
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| 2. |
Tests for jadeite
vs. nephrite |
| 2.1 |
Observation:
In general, nephrite is an aggregate of microscopic
fibrous crystals; hence a weaker lustre than that
of jadeite. |
| 2.2 |
Hardness:
The Chinese name of jadeite is hard jade and nephrite
is soft jade. In other words, jadeite can
scratch
nephrite; however, this destructive test is not
applicable to jade jewel in gemmology. |
| 2.3 |
Hydrostatic
weighing method: If the jewel is not set in ring
or other matter/material, to test the SG is a convenient
method. |
| 2.4 |
Use
refractometer to obtain the RI figures for comparison
(see table above) |
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| 3. |
Tests
for B Jade |
| 3.1 |
Observation:
(1) B jade has a weaker lustre (waxy) than most
types of natural jadeite but the green colour is
sometimes too bright with that lustre. (2) Under
magnification, the crystal edges are fuzzy because
of the solution and destruction of individual
grains after acid bath |
| 3.2 |
Infra-red spectrum: Infra-red spectroscopy
can detect the existence of resin in the tested
sample |
| 3.3 |
Fluorescence:
The resin may exhibit bluish white fluorescence
under long wave ultra-violet light |
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| 4. |
Tests
for C Jade |
| 4.1 |
Observation: Under 10X lens, green colour is seen
being concentrated at grain edges because the dye
stuff occupies the space among individual crystal
grains. |
| 4.2 |
CCF:
If the dye stuff is (most often) chromic oxide to
give green colour, the C Jade appears red when being
viewed under a Chalser Colour filter |
| 4.3 |
Absorption spectrum: The natural jadeite may
have an absorption line at red region (existence
of chromium, 640-650 nm) and blue-violet (existence
of iron, 450 nm). The C Jade has a wider band
at red. |
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| This
issue of Geobits was contributed by Mr. C.H Chow |
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Thank
you for your visit. |
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