Garnet, lapis class lecture

1. GARNET:
Reddish colored garnet is one of the most common gems used in
Jewellery. However garnet need not always be in shades of red,It
could also be colourless, yellow, orange, green and even black.
The deep lavender - rose amethystine rhodolite
Green grossular and demantoid are highly valued gemstones.
Demantoid especially forms a rare & very important gemstone.
The shades of colour in garnet depend on the chromophoric
elements such asFe2+/3+
, Mn2+
,Cr3+
and Ti4+
ions. Pure members of
grossular and Pyrope are colourless.
Though colorless grossular is found in nature, it is doubtful whether
pure uncoloured Pyrope exists.

2. SPECIES COMPOSITION SG RI
Pyralpsite sub
group
Pyrope Mg3Al2(SiO4)3 3.582 1.714
Almandine Fe3 Al2(SiO4)3 4.318 1.830
Spassartine Mn3Al2(SiO4)3 4.190 1.800
Ugrandite sub
group
3.594 1.734
Grossular Ca3Al2(SiO4)3 3.859 1.887
Andradite Ca3(Fe3+
,Ti)2
(SiO
4)3
3.90 1.86
Uvarovite CaCr2(SiO4)3 ~3.40 ~1.71
Hydrogrossular Ca3Al2Si2O3(SiO4
)1 (OH)4m

3. Garnet has strong power of crystallization and generally occurs as well
developed crystals or in ball like forms.
Dodecahedron and trapezohedron or a combination of both is the
common forms of garnets.
In many cases however, owing to subsequent deformation. Crystals
are fractured and occur as shattered pieces. Thus most gem garnets
do not show pure crystal forms.
Garnet crystallizes in the isometric system. It has a hardness of 6.5 to
7.5. Specific gravity lies between 3.58 and 4.32. Cleavage is lacking,
luster is vitreous to resinous. Members of garnet group exhibit wide
variation in RI ranging from 1.70 to 1.89. Hydrogrossular and
demantoid occupy the extreme ends in the RI scale. Many garnet
shows weak anomalous birefringence.

4. PYROPE:
A notable mount of iron is always present in Pyrope. Red colour of
the pyrope is due to the presence of Fe and Cr.
Mg and Fe in the ratio 2:1 produce very attractive rose-red or
rhododendron. Pink to purple colored rhodolite (RI 1.76 and SG
3.84)
Cr along with Fe results in a more pleasant hue that can resemble
ruby and red spinel.
Ruby can be distinguished from Pyrope by its birefringent nature. RI
of red Spinel is 1.730. while that of red Pyrope is more than 1.730
usually around 1.745.
Garnet consisting of 54% Pyrope and 37% Spessartine and traces of
Cr and V has been reported from Tanzania (RI 1.765 & SG 3.88). It
appears blue green in daylight and purple red in incandescent light.

5. Unlike ruby and red Spinel, Pyrope does not show florescence. This is
due to the presence of Fe which quenches the fluorescence.
The existence of Cr can be determined by 687nm and 685nm absorption
lines (sharp doublet like are other Cr bearing stones by using
spectroscope). The broad band at 575-525nm are caused on account of
Fe. Pyrope may have inclusions of stumpy or needle like crystals.
Rhodolites from Orissa posses’ typical hallow cracks or dark radiating
cracks around radioactive minerals. Fine quality pyrope is found in
Kimberlites and other ultra basic rocks. In Kimberly are, S Africa, It is
referred to as cape ruby often it is also called as Tangnyika Ruby.
Arizona Ruby and Elie ruby.
The first gem pyrope comes from Czechoslovakia and is called
Bohemian Garnet. It is found in a mass of Peridotite and basaltic
breccias.
Rhodolite mined at be border of Kenya- Tanzania is known for its
beauty. The other places of occurrence of pyrope are Arizona and
Carolina states of USA. Russia, Myanmar and Brazil.

6. PYROPE

7. ALMANDINE:
The deep reddish colour is due to higher concentration of Fe. RI & SG
are much higher than most other Garnets. A tinge of violet red is
characteristic of Almandine.
It can also be brownish-red. Dark coloured varieties appear almost
black when deeply cut. Such garnet has to be cut shallow to reveal its
colour. Thailand rubies and purplish red Spinels may by mistaken for
almandine.
The RI of almandine is around 1.81.
SG is about 4.0, fluorescence is lacking in Almandine.
Apatite, Magnetite, hornblende and other inclusions are common in
Almandine.

8. Fibrolite/tremolite fibers may also be present. Crystallographically
oriented needles of Rutile produces "silk" when cut into a cabochon,
it results in 4 rayed star intersecting at oblique angles. Usually garnet
requires a strong light source as the effect is rather faint.
Almamdine it reported from all continents. It occurs in regionally
metamorphosed pelitic rocks and occasionally in granitic pegmatite
and rarely in other igneous rocks.
India is the largest producers of almandine. It is mined in Rajasthan,
Orissa, AP, TN and Karnataka.

9. ALMANDINE

10. SPESSARRTINE
Yellow to orange -- brown Spessartine is a much less common gem
than other species or Garnet. It look, more like the Hessonite variety
of grossular and can be mistaken for many orange- yellow birefringent
stones
The RI is around 1.80 and SG is about 4.18. Prominent absorption
bands are seen at 462, 432, 424 and 412nms. Two narrow lines may
also be present at 495 and 485nm.
Those with higher Fe reveal a band at 505nm as well.
Lace like or wavy liquid feather inclusions are distinctive of
Spessartine.
It is non fluorescent under UV light. Spessartine occurs in granitic
pegmatite, skarn and thermally metamorphosed manganous
sediments.
Gem mineral is found in Myanmar, Srilanka, Madagascar,Brazil ,
AustraJia, Sweden and USA.

11. SPESSARTINE

12. GROSSULAR
Grossular of orange shale is better known as cinnamon stone,
hessonite or essonite in Europe and Gomed or Gomedika in India.
The colour ranges from orange yellow to orange brown. Apart from
these shades yellow, brown, purple, orange-red, pink and shades of
green colour are also seen. Pare grossular however is uncolored.
The green colour is due to Cr present in appreciable amount. This
variety emits of H2
0 up to 5% and hence it is called hydrogrossular,
hydrogarnet, Grossularoid and Garnetoid.
The massive green Grossular from Transvaal, South Africa has been
named as Transvaal Jade. Tsavorite or Tsavolite is another bright to
dark green variety of grossular which consists of considerable amount
of V along with Cr.

13. GROSSULAR

14. Vesuivianite, apatite and calcite are the common inclusions in
grossular. Magnetite and Sphene may also be present. Under low
magnification inclusions may appear like droplets oily appearance of
the interior of the stone is characterized of Hessonite.
RI= 1.735 to 1.750 & SG= 3.61 to 3.75. In hydroglossular variety, RI =
1.74 and SG= 3.68
Grossular has no distinct absorption lines but for few weak bands
due to Cr (630) and Fe (461 and 570nm) and Mn content. Both
hydroglossular and Tsavorite may appear red through Chelsea filter
due to Cr content. Green grossular may appear strikingly similar to
emerald, Cr Diopside and Cr tourmaline.

15. The massive green hydroglossular may be mistaken for Jade, Idocrase
and Serpentine. Similarly Hessonite may be confused with many
birefringent stones such as Zircon, Topaz, Sapphire and Heliodar and
Spessartine.
However, all these stones differ in their physical and optical
properties.
Demantoid and spessartine have much higher RI and SG.
Grossular is generally found in thermally metamorphosed impure
calcareous rocks.
Calcium metamorphosed impure calcareous rocks. Calcium
metasomatism can also produce glossular. Zeolite cavities in
metamorphosed basaltic lavas are known to contain grossular
pneumatolysis associated with granite Pegmatites is another source
for this stone. Occasionally it may be formed in Anorthosite layered
complexes. Intruduction of H2O and Al into calcsilicate assemblages
produce hydrogrossular.

16. Srilanka produces fine Hessonite from its gem gravels Brazil,
Canada and Russia (Siberia) are the other important countries.
Pink grossular known as Rosolite, is found in Kenya and the
neighbouring areas of Tanzania is the main source for Tsavorite.
Hydroglossular of various colors is obtained from Transvaal
region of South Africa and USA. Pakistan samples posses brilliant
green resembling Jadeite.

17. ANDRADITE: Andradite is the Fe3+
bearing species of garnet. It can
have a considerable amount of grossular molecules. There are many
varieties of andradite whose color ranges from black to red, brown,
yellow and green.
On account of its beauty, Cr- bearing green variety, Demantoid is the
most talked about variety of andradite, though it is very scarcely
available in nature. Dark brow to black melanite variety consists of 1-
5% TiO2
and in Black schorlomite, TiO2
may go upto 20%. Zr bearing
variety with ZrO2
up to 29% is named as Kinzeyite or Zirconium garnet.
Goldmantite is the vanadiferous variety having V2
O3
up to 18.3%.
Topazolite is honey yellow to greenish yellow and fine grained massive
light brown one is called Allochroite.

18. ANDRADITE

19. The term polyadelphite is applied to a yellowish brown to liver brown
massive variety. Gem andradite is mainly demantoid or Topazlite.
Often black melanite appears lovely when facetted, exhibiting
metallic luster. Among garnets, Andradites has the maximum RI
which can be as high as 1.89 in demantoid and its SG is about 3.85.
the most distinctive feature of the gem is its dispersion of 0.057
Which is much higher than most other natural stones but it does not
appear to be spectacular because of its strong body color.
Demantoid shows characteristic absorption doublet between 699 and
70Inm (Cr) and intense band at 433nm (fe3+
) which merges with the
general absorption of violet due to Cr.

20. These may be fainter bands at 640 and 621nm. Under Chelsea filter it
appears red.
Demantoid may be confused for hydroglossular, chrome diopside,
peridot, green zircon, sphene, green diamond and so called Yitrium
aluminium garnet (YAG).
The RI, SG and absorption lines of all three stones are quite distinct
from Demantoid.
However, except hydroglossular, peridot and Cr bearing diopside, the RI
of other stones including Demantoid goes beyond the ranges of
conventional refractometer and many of them are isotropic as well.
The most distinctive features of demantoid is the inclusions of slender
fibers of asbestos (Byssolite variety of actinolite). Frequently these
fibers radiate from a center. This type of inclusions are referred to as

21. Andradite forms in thermally metamorphosed impure calcareous
sediments and metamorphosed skarn deposits.
Topazlite and Demantoid occur in serpentinites and chlorite schists.
The main source of demantoid is the Ural mountains of Russia.
Topazolite is found in Italy and Switzerland.
Melanite and Schrol amite are found in the under saturated alkaline
igneous rocks like nephline syenites and ijolite.
In India, Melanite occurs in Ijolites of Ambadonngar- Kadipani area
in this section, it often shows twinning and zoning.

22. URAVOITE:
Perhaps among gem garnets the least described is Uvarovite.
Cr- content of Uvarovite can be up to 28%. Uvarovite has a bright
green colour.
The best green colored varieties are found in association with
Chrome deposits of Ural, Russia.
Other sources are found in Finland, Italy and Turkey. It is found in
metamorphosed lime stones, skarn deposits and ultramafic rocks.
Since it is Cr rich garnet there must be occurrence in
metamorphosed ultra basic complexes, hidden somewhere in the
crust from which beautiful Uvarovite could be obtained.

23. UVAROVITE

24. 1.Gemstones as rocks Lapis lazuli
Almost alone among the gem materials, lapis lazuli is a rock in which
the blue minerals like lazurite, hauyne and sodalite are inextricably
mingled with calcite, diopside, pyrite and other species.
In fine specimens the blue mineral dominates giving the stone an
uniform color though brassy specks of pyrite are almost always to be
detected, both improving the look of the stone and conforming its
gentility.
The S.G of fine pieces of Lapis lazuli is about 2.80 (2.7-2.9). True lapis
can thus be known from its most common stimulant i.e. stained jasper
(industrially known as ‘Swiss lapis’) which has an S.G. of only 2.58.
Also a costly imitation of synthetic spinel embedded with gold flakes
has SG value of 3.52 and shows strong cobalt absorption spectrum at
650 nm (strong), 580 nm (weak) and 532 nm (strong) as well as 480
and 452 nm.

27. Garnet
Garnet is an isometric aluminosilicate normally developing a
dodecahedral crystal form, icositetrahedral form being much less
common.
The principle end member varieties of garnet are pyrope
(Mg3
Al2
Si3
O12
), almandine (Fe3
Al2
Si3
O12
), spessartine (Mn3
Al2
Si3
O12
),
grossular (Ca3
Al2
Si3
O12
), andradite (Ca3
Fe2
Si3
O12
) and uvarovite
(Ca3
Cr2
Si3
O12
).
However, extensive solid solutions are possible between these end
members. Some additional terms for gem variety garnets are:
•Hessonite : Grossular garnet
•Demantoid : Andradite garnet

28. Table lists the properties of the different end member garnets.
Table : Physical and optical properties for the end member
garnet compositions.
Gem variety pyrope is red in color and has a vitreous luster.
They are seldom found in large sizes.
Also their color, though fine, is frequently too dark to be
appreciated. Pyrope can be confused with red spinel but the
constants are different.
The absorption spectrum of pyrope has a dark band at 575 nm
(yellow-green).

29. Variety Refractive
index
Dispersio
n
Specific
Gravity
Hardne
ss
Almandine 1.76 – 1.81 0.024 3.8 – 4.2 7.5
Andradite 1.89 0.057 3.85 6.5
Grossular 1.74 – 1.75 0.028 3.6 - 3.7 7.0 –
7.5
Pyrope 1.74 -1.76 0.022 3.65 – 3.8 7.25
Spessartine 1.80 -1.82 0.027 4.16-4.19 7.25
Uvarovite 1.87 0.030 3.77 7.5

30. Almandine is red in color (sometimes purplish red) and has a
vitreous luster.
Sometimes the color is so deep that it appears almost black. Siam
ruby has almost the same color but can readily be distinguished
by SG and R.I.
Also ruby is doubly refracting whereas isometric almandine has
single refraction. Spectroscope provides an yet more certain test
Almandine doublets are sometimes encountered with a table
facet of almandine and a pavilion of glass. In such cases there will
be a layer mineral inclusions followed by typical gas bubbles. At
times, the inclusions in almandine, oriented parallel to the
dodecahedral faces can produce a weak star effect.

31. Spessartine can range in color from yellow through orange to flame
red.
Often it closely resembles hessonite garnet in appearance though it
lacks the granular inclusions and treacly swirls common in hessonite.
Solid solution with almandine renders wine color to the gem. The
constants are close to almandine but the absorption spectrum can be
useful in distinction .
Grossular (hessonite) occurs in shades of orange-brown or honey-
brown. Under microscope it has a peculiar granular appearance
resulting from numerous small inclusions.

32. A massive green variety of hydrogrossular has been found in South
Africa and misleadingly called Transvaal jade.
Also pink to rose red varieties of this hydrogrossulars are used as
gems. They have an SG range of 3.28-3.57 and sometimes even 3.63.
The R.I. for the later is 1.738.
Hydrogrossular gives orange fluorescence under X-rays.
Andradite is a green variety of garnet and is called demantoid in its
gem variety.
A peculiarity of most of the demantoids is the nature of its inclusions.
These are tiny silky asbestos fibers often radiating from one or more
centers.
Also demantoid can have a vitreous to subadamantine luster and
might appear pink through Chelsea filter.

34. LAPISLAZULI
The most sought after member of this group is lapis lazuli or the
rock rich in lazurite (Na, Ca)8
(AlSiO4
)6
(SO4
,S,Cl)2
RI 1.50, SG 2.40-2.45.
It is idiochromatic gem characterized by ultramarine blue colour.
Rock rich in lazurite is called as lapis lazuli.
Lazurite is the end member of sodalite group. Sodalite crystallizes
in the cubic system, it is a product of thermal metamorphism of
sulphurous impure limestone.

35. It is invariably associated with crystals of pyrite and very often
with veins of calcite.
In thin section lapis reveals deep blue lazurite, colourless
calcite, palegreen diopside, fosterite, opaque pyrite and other
minerals. Specific gravity of lapis may go up to 2.95.
It is believed to be protector from evil forces. There are many
substitutes for lapis like dyed limestone/ marble. Jasper stained
blue is called swiss lapis. However in there stained substitutes,
pyrite inclusions are typically absent.
Opaque blue glass impregnated with copper crystals is
manufactured. Under magnification they reveal gas bubbles and
octahedral nature of copper crystals.
Blue sodalite and hauyne may appear like lapis. There also lack
pyrite crystals and they have different RI and SG, these two may be
slightly transparent, while lapis is completely opaque. Under UV
light, calcite veins of lapis usually florescence to pale pink.

37. Lapis is one of the earliest known gems held in esteem
by Sumerians in Mesopotamia and Harappans in western
India.
The most important mine for lapis is located in the
inhospitable icy passes of firgamu (36° 10': 71° 00'),
Badakhshan in Afghanistan. This is one of the earliest known
mine in the world.

38. For recovering lapis lazuli in marble, the host rock was heated
and water from melted ice thrown on the heated rock resulting in
cracking (Zucker 1984).
The Siberian lapis lazuli from a place near lake Baikal. Russian
lapis has more pyrite and calcite veins and quality wise it is inferior
to Afghan lapis.
A large source of lapis exists north of Satiago in Chile. It has
more greenish tinge. US lapis has deep colour, while Canada lapis
has a lighter shade.
Thus Afghan lapis are esteemed by connoisseurs. Afghan lapis,
because of their pleasing colour were used in painting in earlier
days. Azure blue, indigo blue and artificial copper blue colours
were considered inferior to that of lapis, which has intense royal
and ultra massive blue with touch of slight purple.

39. Many beads of lapis have been recovered from Harappan
sites.
They were either imported from Afghanistan or mined locally.
If imported ,it proves existence of trade between two regions
as far back 3500-5000 year BP.
There are however a few reports of occurrences of lapis in
India (Iyer, 1961):

40. •Lapis with pyrite inclusions from Nagpahar hills, West of Ajmer
which requires the confirmation.
•Near Bamasur hill in Bhandara district , MP, where in Schistose
quartzite is said to have streaks of bright blue lapis and also bands
consisting of whitish light grayish and greenish blue minerals.
•Blue lapis lazuli from Zanskar mountain range in Kashmir.
The existence of mines at the above places has been confirmed.
There is one more confirmed report of the occurrence of the lapis
blue patches in the limestone intruded by granites near Abu road.
since the contact metamorphism of impure limestone could
produce lapis, these patches caused very likely be lapis.
There are many other places in India, where lime stones are
intruded by granite plutons. Possibilities of existence of lapis in India

41. A lapis lazuli bowl
from Iran (End of
3rd, beginning 2nd
millennium BCE)

42. A Mesopotamian lapis lazuli
pendant circa 2900 BCE.

43. In the funeral mask of Tutankhamun (1341-1323 BCE), lapis lazuli was
used for the eyebrows of the young Pharaoh.

44. Close-up of the lapis lazuli inlays
in the 25th-century BCE Statue of
Ebih-Il
An elephant carving on high-quality
lapis lazuli, which includes gold-
colored pyrite, is a rare example
of Mughal inspired art. (length:
8 cm (3.1 in))

45. Carved lapis
lazuli mountain
scene, from the
Chinese Qing
Dynasty (1644–
1912).
Girl with a pearl earring
(1665) by Johannes
Vermeer is painted with
ultramarine, a natural
pigment made from lapis
lazuli.

46. A lapis lazuli urn two meters high from the
State Hermitage Museum in Saint
Petersburg, Russia (19th century).

47. Turquoise
Turquoise is an idiochromatic opaque mineral containing
copper, it was regarded as stone of sky gods. Blue colour is due
to copper. Turquoise is the hydrated phosphate of copper and
aluminium (Cu Al6
(OH)8
(PO4
) 4H2
O). It crystallizes in triclinic
system. The name is derived from ‘Turkey’ although these
stones were never found, but they are traded.

48. Turquoise used as the gemstone in reality is not a single mineral, but
represents a group of six members:
planerite(Al), turquoise (Cu, Al), Faustite(Zn,Al), aheylite (Fe+2
,Al),
Chalco siderite (Cu, Fe+3
), and an unnamed Fe+2
-Fe+3
analogue.
Turquoise is one of the stone that never occurs in transparent form; it
can be translucent at the edges. Its luster is waxy, hardness 5-6; SG 2.6
to 2.83; RI α-1.61 β- 1.62 ν- 1.65, δ- 0.04; extinction angle 12°. The
colour of the synthetic turquoise is paler and SG is 2.7 natural stones like
chalcydony and cherts are dyed to imitate turquoise. Some naturally
coloured stones also resemble turquoise.

49. Turquoise can be tested by following ways
•1.Its waxy luster on the broken surface.
•2.Absorption spectra
•3.Dipped in HCl, the fragments of turquoise colour the flame
initially into blue and further to green.
•4.When ammonia is added to turquoise dissolved in HCl, the
solution turns blue, but not its imitations. Those treated with copper
salts turn yellow green immediately after adding HCl.
•5.Hot point treatment to detect impregnation or banding which
melts wax and resin. Addition of CCl4
removes the paraffin wax or oil.
•6.When heated in the test tube, stone treated with wax and plastic
burn with characteristic odour.

50. Test 3,4,5 and 6 are destructive and only useful in testing rough
stones. With caution tests, 4 and 5 may be carried out on some in
inconspicous part at the back of the cut stone.
Soon after the HCl – ammonia treatment stones have to be
thoroughly washed. Specimens bounded by using sodium silicate
do not react to tests 5 and 6.

51. Turquoise is the mineral of secondary origin and is found in various
rock types that have undergone extensive alterations. The
decomposition of apatite, aluminous minerals and small amount of
copper minerals may result in deposition of turquoise from
phosphoric acid in vein, cavities or as lumps within the parent rock.
Turquoise from Sarabit Elkhaden –Wady Maghara in Sinai Peninsula
accounts for ancient source. The southern slopes of Ali-mersai,
2021m peak near Nishapur in Iran yield fine quality of turquoise
over several centuries. In this locality it occurs in brecciated
porphyritic trachyte and clay slate sequences.
The greenish blue variety occurs in Karkaralinsk in Siberia, Russia. In
USA it occurs in Nevada, New Mexico, Chile, Australia Turkestan,
France and England.

53. TURQUOISE

55. Animal - Insect - Butterfly - Turquoise, 18th C
turquoise medallion

56. Fall Ball Gown Sweetheart Ruffles and
Beaded Decorate Turquoise
Quinceanera Gowns
Rayban turquoise