Garnet group , Types of Garnet and its identification Techniques.

Garnet

Garnet is a mineral that comes in many different colors, but is most commonly found in shades of red. Its name comes from the Latin word "granatum," which means "seed," because the small red crystals that garnet often resembles the seeds of a pomegranate.

Garnet has been used for thousands of years as a gemstone and has been highly valued by many cultures throughout history. It was believed to have many healing properties and was used to treat a variety of ailments.

In addition to its physical healing properties, garnet was also believed to have emotional and spiritual healing properties. It was said to help with emotional healing, to promote positive feelings such as love and compassion, and to help release negative emotions such as anger and jealousy.

The garnet group comprises six species, which tall into two series known as the PYRALSPITE series and the UGRANDITE series. 

Chemistry: The six species have the general formula L3M2(SiO4)3. In the PYRALSPTTE series, L may be Mg, Fe or Mn; M is always Al. In the UGRANDITE series, L is always calcium; M may be Cr, Al or Fe. Crystal system and habit: Cubic: Often very perfect crystals. The rhombic dodecahedron and trapezohedron are highly characteristic. When water-worn in gem gravels, the crystals may be almost spherical.

spessartite garnet with quartz 

Garnet

Varieties:

Pyralspite Group:

Pyrope (Cape-Ruby) : Transparent to sub-translucent. Red, orangy-red, purplish-red. 

Rhodolite: (Mid-member between pyrope and almandine) purplish-red or pink to violet. 

Almandine (Carbuncle) : Transparent to sub-translucent. Brownish-red to purplish-red.

Phenomena: Some almandine may display a weak 4 or 6-rayed star. 

Spessartine: Yellowish-orange, red, brownish-red.

 

Malaya: (Mixture of pyrope and spessartine) Tp, pinkish-orange through yellowish-orange. 

Phenomena: A colour-changing garnet (colour variable) consists of a mixture of pyrope and spessartine).

Ugrandite Group:

 Uvarovite: Idiochromatic, colour due to chromium. Attractive 'emerald' green, but crystals suitable for fashioning is extremely rare making it almost unknown as a gemstone.

 

Grossular: Transparent to translucent

 Hessonite: (Cinnamon-Stone, Hyacinth, Jacintti) Brownish-yellow to brownish-red, yellowish-orange.

Origin: Le Chang, Guang Dong Province, China

Sample size: 5 x 5 x 2.7 cm

Origin: Le Chang, Guang Dong Province, China

Sample size: 5 x 5 x 2.7 cm

Origin: Le Chang, Guang Dong Province, China

Sample size: 5 x 5 x 2.7 cm

Tsavorite: Bright bluish-green to yellowing green. Yellow and colourless are also known. (R.I.'s slightly lower.)

Tsavorite

Origin: Merelani Hills, Arusha, Tanzania

Sample size: 1.2 x 1 x 0.7 cm

Tsavorite

Origin: Merelani Hills, Arusha, Tanzania

Sample size: 1.2 x 1 x 0.7 cm

Tsavorite

Origin: Merelani Hills, Arusha, Tanzania

Sample size: 1.2 x 1 x 0.7 cm

Tsavorite

Origin: Merelani Hills, Arusha, Tanzania

Sample size: 1.2 x 1 x 0.7 cm

Hgdrogrossular Translucent to sub-translucent, green, pink, greyish-white ornamental.

Andradite: Transparent to sub-translucent. - Demantoid: Green; rarely yellow (Topaziolite'). - Melanite: Black, nearly opaque.

Demantoid Garnet from Ghazni, Afghanistan.

Demantoid Garnet from Ghazni, Afghanistan.

Phenomena: Rare yellow demantoid may show chatoyancy. Some very dark semi­translucent  to opaque andradite Crystals show iridescence

Demantoid Garnet from Ghazni, Afghanistan.

Physical properties:

Table 15.2 Physical properties of the garnet group.

	Pyrope	Almandine	Spessartine	Grossular	Andradite
Cleavage	none	none	none	none	none
Hardness	7.00-7.50	7.00-7.50	7.00-7.50	7.00-7.50	6.50-7.00
Specific Gravity	3.70-3.90	3.90-4.30	4.12-4.20	3.57-3.73	3.81-3.87
Refractive Index	1.72- 1.76	1.77- 1.82	1.79- 1.814	1.73- 1.76	1.855 - 1.895
Optical Nature	S/ R	S/R	S/R	S/R	S/R
Dispersion	0.002	0.24	0.27	0.028	0.057
Lustre	bright vitreous	bright vitreous	bright vitreous	bright vitreous	sub­ adamantine

 

Other points:

Durability: Good

    Birthstone: January

Pyralspite Series:

 

 

Stones are SR but ADR is common, particularly among the reds.

 PYROPE: Red, orangy-red, purplish-red.

 Inclusions: Low relief and acicular crystals.

Spectrum: Chrome present in best colours; most show presence of some almandine.

Occurrence: South Africa, Sri Lanka, USA, Australia, Russia.

RHODOLITE: (Light. to dark reddish-purple to purplish-red). Mid-member between pyrope and almandine. No internationally accepted name has been found for this section of the series (R.I. l.74-1.77/S.G. 3.74-3.9(Usually shows almandine spectrum). 

Occurrence: Sri Lanka, Tanzania, Zimbabwe. Madagascar, USA.

ALMANDINE: Idiochromatic, colour is due to iron. Some almandine displays a weak 4- or 6-rayed star. The name 'carbuncle' (small coal) is still sometimes applied to almandine cut in the form of hollow cabochons (to lighten colour look).

Inclusions: Needles intersecting (at 110" and 70°) in two (occasionally 3) directions, which may give rise to a 4 or 6-rayed star: both well-formed and rounded crystals; zircon halos.

    Spectrum: Colour is due to iron. Typical spectrum has bands at 505,        527 and 575 nm. A series of weaker bands may be seen on either side.

                             Almandine Garnet Spectrum

                             Almandine+ Spessartine Garnet

Occurrence: India, Sri Lanka, Brazil, Madagascar, Tanzania, Zambia, Pakistan. USA.

MALAYA (Mali): is a bright orange garnet and a member of the pyralspite series (R.I. 1.75 to 1.76) and a rare colour-changing garnet (bluish-green in daylight/purplish-red in incandescent) with an R.I. of 1.765, S.G. 3.88, consists primarily of a mixture of pyrope ­spessartine garnet.

Occurrence: East Africa.

SPESSARTINE: Colored by manganese. Yellowish-orange, red, brownish-red.

Inclusions: Wavy liquid droplets, and 2-phase inclusions. 

       SPESSARTINE Garnet Spectrum

Occurrence: Sri Lanka, Africa, Namibia, Brazil, Burma, Madagascar, India, Australia, USA

Ugrandite Series:

 GROSSULAR: Transparent to translucent.

HESSONITE: Inclusions: Classic granular appearance caused by a profusion of rounded crystals (may he. apatite or zircon) and oily/treacly internal effect.

Occurrence: Gem gravels of Sri Lanka, Brazil, Canada, Kenya (relatively clean).

TSAVORITE: Color is attributed to the presence of vanadium but many stones appear red under 'the Chelsea Color Filter due to a trace of chrome. Colourless and yellow grossular R.Is are slightly lower at 1.750 to 1.762

Occurrence: Kenya (Tsavo ), Tanzania, Pakistan.

HYDROGROSSULAR: Translucent to opaque green and pink massive material. Chromium is the cause of colour in some green varieties (may appear pinkish through the Chelsea Colour Filter). 

Inclusions: May show black inclusions in translucent pink or green background.

Occurrence:, New Zealand, USA, Pakistan

 ANDRADITE: 

DEMANTOID: Green, yellowish-green, may appear reddish under the CCF. Yellow demantoid very rare. Stones are usually less than 1 carat. Inclusions: Classic 'horsetail' (fibrous byssolite). 

Occurrence: Green -  (Urals - found as rolled pebbles, fine green material now rare), Italy, Zaire. Yellow - Italy, Switzerland. Pakistan

 

Horsetail Inclusions in Demantoid garnet  Origin: Khuzdar district, Baluchistan Province, Pakistan.

Horsetail Inclusions in Demantoid garnet  Origin: Khuzdar district, Baluchistan Province, Pakistan.

Horsetail Inclusions in Demantoid garnet  Origin: Khuzdar district, Baluchistan Province, Pakistan.

Horsetail Inclusions in Demantoid garnet  Origin: Khuzdar district, Baluchistan Province, Pakistan.

Horsetail Inclusions in Demantoid garnet  Origin: Khuzdar district, Baluchistan Province, Pakistan.

Horsetail Inclusions in Demantoid garnet  Origin: Khuzdar district, Baluchistan Province, Pakistan.

Horsetail Inclusions in Demantoid garnet  Origin: Khuzdar district, Baluchistan Province, Pakistan.

Horsetail Inclusions in Demantoid garnet  Origin: Khuzdar district, Baluchistan Province, Pakistan.

KEY POINTS

 

1.    Almandine from Pyrope                                  RI, Spectra

2.    Almandine from Rhodolite                               RI, Colour

3.    Almandine from Malaya                                   RI, Spectra, Color change

4.    Almandine from Spessartine                           Colour, spectrum

5.    Almandine from Hessonite                              Colour, RI, Spectrum

6.    Rhodolite from Pyrope                                     Colour, RI, Spectrum

7.    Rhodolite from Malaya                                     Colour, Spectrum, RI

8.    Pyrope from red spinal, syn. Spinal                 Magnification, RI, F

9.    Pyrope from Hessonite                                    colour, spectra ,RI,F

10. Malaya from Hessonite                                     Colour, spectra, RI

11. Demantoid(green) from Tsavorite (green)        RI, Spectrum, Magnetic

12. Spessartine(orange) from Hessonite               RI, Spectra

Note: If the garnet RI is 1.740 to 1.770, and purple to purples to red or     pink colour and an almandine spectrum, it should be identified as       rhodolite garnet