OBSIDIAN :

Etym. From the Latin ‘obsidiannus lapis’ stone brought back from Ethiopia by Obsidius

A natural volcanic glass; amorphous

Physical and optical and properties :

- Colours : brown, yellowish reddish, bluish, greenish, bottle green, grey, black

- Transparency : transparent to opaque

- Lustre : vitreous

- Hardness : 5 to 6

- S.G. : 2.35 to 2.60

- R.I. : 1.48 to 1.60

(BANDED) OBSIDIAN :

Obsidian presenting bands of different colours

OBSIDIANITE :

Synonym for tektite .

OBSIDIENNE :

French for Obsidian .

OCCIDENTAL CAT’S EYE :

Prohibited Appellation for a Quartz cat’s eye

OCCIDENTAL RUBY :

Very old term sometimes used for a true ruby to differentiate it from spinel .

OCCIDENTAL TURQUOISE :

Prohibited Appellation for odontolite .

OCTAEDRITE :

Synonym of anatase .

OCULUS MUNDI :

Etym. From the Latin, meaning = eye of the world

Name given to a variety of hydrophane presenting a play of many colours

ODALITE :

Synonym for sodalite .

ODONTOLITE :

Signifies : fossil tooth, synonym of false turquoise

is constituted of fragments of tooth or of fossil bones penetrated by iron containing phosphates and by phosphates of aluminium

CaCO3 +

Physical and optical and properties :

- Colours : grey-blue, coloured blue by iron-phosphate, serves as an imitation for turquoise

often artificially coloured to imitate turquoise

- structure like teeth of a saw

- Transparency : opaque

- Hardness : 5

- S.G. : 3 to 3.5

- R.I. : 1.60 to 1.64

OEIL DE CHAT :

French for Cat’s eye

OEIL D’EPRERVIER:

Term existing only in French. Name used for a variety of tiger’s eye Quartz of red colour  

OISANITE :

from Delamétherie : synonym of anatase .

from Klaproth: synonym of axinite .

from De Saussure: synonym of Thallite or of Delphinite .

OLAFITE :

Variety of albite

OLIGISTE :

French name, synonym for hematite

Fe₂0₃

OLIGOCLASE :

Stone belonging to the group of the feldspars comprising the sunstone or also called aventurine feldspar and exceptionally a colourless to pale blue specimen from North Carolina, USA ( for the latter see oligoclase moonstone)

Colours : usually orange to brown with sparkling optical effect (called aventurescence) due to reflection on small hematite or goethite platelets as inclusions

- Hardness 5 to 6

- S.G. : 2.6 to 2.7

- R.I. : 1.54 to 1.55

- Birefringence : 0.007

- System : triclinic

OLIGOCLASE MOONSTONE :

Variety of oligoclase showing adularescence; of white, greyish or bluish colour and found in North Carolina, USA

OLIVENITE :

Hydrated Arseniate of Cu, of green colour, orthorhombic system, rare gemstone

OLIVINE :

Synonym of peridot or Prohibited Appellation for a demantoid garnet .

OLIVINE from the URALS :

Prohibited Appellation for a demantoid garnet.

OLYTHOLITE :

Synonym for grossular garnet .

ONEGITE :

Sagenitic Quartz with slight amethyst colour coming from near the Onega Lake, North of Leningrad

or variety of Quartz with goethite inclusions

ONICE :

Spanish for onyx .

ONICOLO :

Synonym for nicolo .

ONTARIOLITE :

Variety of scapolite .

ONYCHITE :

Name given in ancient times to an ornamental stone, Alabaster or calcite, with yellow or brown veins, employed for the manufacture of vases

ONYX :

Etym. from the Greek: onyx = nail

Onyxes were supposed to come from de nails of the Goddess Aphrodite that she cut and that were thrown into the river Indus.

Name given to a variety of banded agate, with straight and parallel lines generally of black and white colour, also white and grey , when white and brown or reddish brown called sardonyx , when white and red called carnelian-onyx

Also the term designating uniformly black coloured chalcedony; often obtained by ‘burning’ common agate

Equally employed for designating an artificially tinted banded agate

Prohibited Appellation for designating marbles or other ornamental banded stones used for manufacturing small ornamental objects (especially the so-called onyx marble)

Prohibited Appellation for jet

(ALGERIAN) ONYX :

Prohibited Appellation for a variety of calcareous alabaster

(BLUE) ONYX :

Prohibited Appellation for agate or chalcedony that are artificially coloured

(BRAZILIAN) ONYX:

Prohibitted Appellation for a marble of superior quality and colour coming actually from Argentina

(CALIFORNIAN) ONYX :

Prohibited Appellation for a veined marble; or for calcite yellow, pink or green in colour

(EGYPTIAN) ONYX :

Prohibited Appellation for a variety of aragonite .

(GREEN) ONYX:

Prohibited Appellation for a chrysoprase or for an artificially green tinted agate

(MEXICAN) ONYX :

Prohibited Appellation for an onyx marble

ONYX ALABASTER :

Prohibited Appellation for a banded calcite

ONYX ALBATRE :

French for onyx alabaster

ONYX MARBLE :

A rock, compact and translucent; it is a limestone composed of Calcite or Aragonite, showing more or less parallel bands that are differently coloured and generally curved

It is formed by layered (so always banded) deposits of hydrated calcium carbonate from a hot spring, or as stalactites and stalagmites in caves

Colours : White, yellowish , greyish , brown to brownish or reddish, sometimes greenish-olive.

- Transparency : translucent to opaque

- Lustre greasy

- Hardness : 3

- S.G. : 2.7

- R.I. : 1.486 – 1.658

ONYX OBSIDIAN :

Synonym for banded obsidian

ONYX OPAL :

Common opal having parallel bands

OPAL :

Etym. from the Sanskrit : Upala = stone

in Latin: opalus signifies: precious stone

SiO₂ + H₂0 hydrous silicon dioxide ; amorphous

Physical and optical and properties :

- Colours : white, colourless, yellow, brown , grey, green-blue , black, with play of colours covering the whole range of colours of the solar spectrum ( called opalescence )

- Transparency : transparent to opaque

- Hardness : 5 to 6.5

- S.G. : average 1.90 to 2.30 (1.25 to 2.30 )

- R.I. : 1.37 to 1.47

- Fracture : conchoidal, splintery, brittle

See also Gemmological News concerning opal.

Or the different articles concerning opal.

(AGATE) OPAL :

Variety of opal banded like agate or presenting alternating layers of opal and layers of chalcedony

(AUSTRALIAN) OPAL :

Opal coming from Australia , but generally designating its variety of black opal

(AMBER) OPAL :

Variety of opal having the colour of yellow amber

(BAMBOO) OPAL :

It is a mass of silica found in the stems of the bamboo : colour tobacco blond, and presenting the characteristics of opal

(BLACK) OPAL :

Natural opal designating generally all opals with a nice play of colour on a black to very dark brown background

Prohibited Appellation for a doublet made of white opal glued on a black onyx or any other black material

(CELESTIAL) OPAL :

Prohibited Appellation for a moonstone .

(CHERRY) OPAL :

Variety of fire opal from Mexico , translucent, of cherry red colour 

(CHINESE) OPAL :

Prohibited Appellation for a chalcedony

(COMMON) OPAL:

Opal having basically only a mineralogical value, containing but very little or none at all precious opal

(FIRE) OPAL :

Variety of opal almost exclusively coming from Mexico , that does not generally have an opalescent effect, does not show a play of colours

- Colours : yellow, yellow-orange, brown-red, red

- Hardness : 6

- R.I.: 1.45

- S.G. : 2.0

(FUGITIVE) OPAL :

Variety of opal showing red flames all of a sudden that disappear as soon as they appeared.

(GIRASOL) OPAL :

Synonym of water opal .

(GLASS) OPAL :
German name for Hyalite opal or for crystalline salt.

(GOLDEN) OPAL :

Opal presenting only a golden yellow colour

(HARLEQUIN) OPAL :

Variety of Opal, generally on white background, containing angular coloured spots resembling a mosaic.

(HYALITE) OPAL :

Colourless transparent opal that shows but very little play of colours.

(HYDROPHANE) OPAL or HYDROPHANE :
Variety of very porous Opal, opaque in air and transparent immerged in water.
Physical and optical properties :
Idem to opal.
Remarks :
The play of colour of hydrophane is only remarked when immerged in water.

(LECHOS) OPALE :

French name. See Lechosos opal .

(MATRIX) OPAL :

A gemstone showing veins of opal in rocky gangue

(MENILITE) OPAL :

See Menilite .

(MILKY) OPAL or MILK OPAL :
English Name for an opal with a hazy or creamy milky appearance

(MOSS) OPAL :

Milky opal with black inclusions resembling to moss

(NOBLE) OPAL :

Natural opal, designating generally all opals with a nice play of colour on a white to milky white background

(ORIENTAL) OPAL :

Prohibited Appellation for a moonstone

(PINTAS) OPAL :

Mexican name for opals showing a particularly brilliant spectrum of colours

(PRASE) OPAL :

Variety of common opal of green colour

(WATER) OPAL :

Precious opal, transparent and colourless material displaying its play of colours at best after being soaked in liquid

(WHITE) OPAL :

Term designating a variety of Australian opals from White Cliffs , in order to differentiate them from the black opals

(WOOD) OPAL :

Fossilised wood that is also opalised ( the molecules of the wood are here replaced by opal ), conserving the structure of the wood, not to be confounded with the regular silicified fossilised wood

OPAL DOUBLET :

Imitation of opal composed of a thin layer of noble natural opal and reinforced with a base of common opal, of onyx or even of glass (or any other suitable material)

OPALE DE FEU :

French for Fire Opal

OPAL from CEYLON :

Prohibited Appellation for a moonstone

OPALINE :

- Synonym for impure matrix opal, or name formerly given to opals from Australia in general

- Opal pseudomorphous after serpentine

- Name given to a pale blue to bluish white girasol sapphire

- Artificial glass generally with a composition based on fluorides, of blue tint and opalescent

OPALITE :

- Term employed to designate a variety of impure common opal, sometimes used as synonym of myrickite.

- Name erroneously given to cinabar in chalcedony

- Black opalescent glass manufactured for opal doublets

OPAL NOBBIES :

Variety of black opals from Australia  

OPAL TRIPLET :

Imitation of opal formed of three parts, the upper part is rock crystal, the middle part is a natural opal and the inferior part is a black glass ( or an onyx , or any other suitable material).

The black glass has a R.I. of 1,49

The S.G. of the whole stone is about 2,62 

OPHICALCITE :

Variety of a rock composed of serpentine, mixed with calcite, dolomite and magnesite

With a cloudy aspect, white to pale green, going to medium green

- Hardness : 3

- S.G. : 2.48 to 2.77

- R.I. : 1.56   

OREGONITE :

Variety of orbicular jasper characterised by white circles around the spherules .

OPHICITE :

Synonym of ophicalcite .

OPHITE :

Variety of serpentine

OPHTHALMIUS :

Medieval name for opal

ORIENTAL CAT’S EYE :

Old Appellation for a chrysoberyl cat’s eye

ORIENTAL OPAL :

Prohibited Appellation for a moonstone.

ORIENTAL SAPPHIRE :

Very old Term sometimes used to designate a true sapphire .

ORIENTAL SUNSTONE :

Prohibited Appellation for a girasol corundum, reddish or yellowish in colour.

ORIENTAL VERMEILLE :

Prohibited appellation for a brownish red corundum .

ORNYCHIPUNETE :

According to Plinius a variety of jasper with clouds or snow flakes

ORSYTAL :

A synthetic ruby of pidgeon blood colour

ORTHITE :

Synonym of allanite .

Ethym. From the Greek orthos : straight ( due to its very straight crystals) ; allanite belongs to the group of the epidotes

Colours : brown to black

Hardness : 5.5 to 6

ORTHOCLASE :

Etym. from the Greek: orthos : straight ( adj. )

and the Greek : klas : that splits easily

potassium feldspar

Si₃AlO₈K

Physical and optical and properties :

- Colours : colourless, golden yellow, light yellow, orange, light pink, grey, creamy, green , bluish

adularia (moonstone ) is one of the near colourless orthoclase varieties, sanidine is a brown one

- Transparency : transparent to translucent

- Hardness : 6 to 6.5

- S.G. : 2.54 to 2.62

- R.I. : 1.518 – 1.526 (+ 0.004)

- Birefringence : 0.008

- Dispersion : 0.008

- Biaxial negative

- System monoclinic

ORTHOITE :

Synonym of allanite.

ORTHOPYROXENES :

Group of pyroxenes crystallising in the orthorhombic system : enstatite, hyperstene .

ORTHOSE :

Orthose is a yellow orthoclase

OSTEOLITE :

Variety of impure and massive apatite

OSTRANITE :

Altered Zircon

OUTREMER :

Ancient (French) Name for lapis lazuli used as a pigment (colour ‘bleu outremer’)

OUVAROVITE :

See Uvarovite .

OUWAROWITE :

See Uvarovite .

OVO DOEMA :

Brazilian term for a quartz crystal worn down by water .

OZARKITE :

Variety of massive thomsonite from Arkansas