Spinel (MgAl2O4) is a much maligned gem mineral that is often thought of as a corundum simulant because in red and blue varieties it can closely resemble ruby or sapphire. It is, however, an outstanding gem material in its own right; it is hard (8), possesses an intermediate to high R.I., shows good dispersion, and is available in a wide variety of colors. Many of the worlds most famous large "rubies" are, in fact, red spinel (e.g. Black Princes' Ruby and Timur Ruby, both in British Crown Jewels). Such stones were once referred to as Balas Ruby, a term that is not used today but is synonymous with red spinel. The confusion is well-founded; the finest red spinels come from the same gem gravel deposits in Myanmar that are renowned for "Burma" ruby, and both share somewhat similar optical and physical properties.

SPINEL DEPOSITS

http://www.gemcal.com/

British Crown Jewels

The Black Prince’s Ruby now is mounted in the front of the Imperial State Crown, just above the famous Cullinan II Diamond. It is a huge, semi-polished octahedron.* Sitwell15 states that the stone is backed by a gold foil, as were many ancient gems, to improve its brilliance. This has not been removed for fear of damaging the gem. The stone measures some two inches (5.08 cm) in length and is of proportionate width.14 Its exact weight is unknown, but estimates put it at ~140 ct.

http://www.rub-y sapphire.co

m/spinel.htm v . 1.00

Spinel, like garnet and tourmaline, is a mineral name that refers to a group of minerals all having the same crystal structure. Members within the group differ by containing varying amounts of Fe, Cr, Zn, Mn and Ni that substitute for Mg and Al in the crystal lattice. Probably the most familiar minerals that have the spinel crystal structure are the ore minerals magnetite (Fe2+Fe3+

2O4) and chromite (Fe2+Cr2O4), neither of which is a gem material. Gem spinel is usually quite close in composition to Mg, Al spinel but contains small amounts of Fe, Zn, and Cr that act as chromophores and account for the wide range of possible colors. Red and pink spinel is colored by trace quantities of Cr; blue, violet, orange and green by Fe. Zn, Cr, and Co (rare) are also present in some blue, violet and purple stones.

Burma (Myanmar)

• Mogok Stone Tract• Source of most of the famous large red spinels; finest red and pink spinel found here. • Source is metamorphosed limestone and its weathering products (gem gravels). • Grey-blue or black material may show 6- or 4-rayed stars. • Magnetite inclusions are characteristic

-http://www.asahi net.or.jp/~u-g7s ktu/english.htm

Pain Pyit, Mogok, Myanmar(MgAl2 O4 )

Parker mine,Notre Dame de La

us,Quebec,Canada MgAl2 O4 Size:1 .9 cm(Phot

o)

Aldan Yakutia,Ru ssia MgAl2 O4

Sri Lanka, Thailand, Cambodia

• Same gem gravels that produce corundum. • Most Sri Lankan spinels are said to be paler (color "more subdued"; "lighter tone") and less included than their Burmese counterparts.

Http://www.thaigem.com/

Table I. A selection of spinels

Compound Characteri sti cs 2 4MgAl O SSSSSS itself, base f or nat

ur al gemst ones 2 4ZnAl O Gahnite , a t r anspar en

t di amagnet i c spi nel 2 4FeAl O Hercynite, a cl assi ca

l par amagnet - gamma Fe2 O3 Maghemi t e , a nat ur a

l mat er i al f or magnet i c r ecor di ng FeCr2 O4 Chromite, t he chr ome

or e of Rhodesi a Mn3 O4 Hausmannite, a nat ur

al t et r agonal spi nel

Compound Characteri sti cs Fe3 O4 Magnet i t e , the ancien t navigator’s lodestone

Fe3 S4 Greigite , a f er r i magne t i c semi met al

2 4NiFe O Trevorite , a f er r i magn et i c semi conduct or

ZnFe2 O4 Franklinite , t he par am agnet i c f er r i t e

CuCo2 S4 Carrollit e, a nat ur al me t al l i c spi nel

Fe2 TiO4 ? , with giant magnetostrictive properties

2 4Mg SiO - The high pressure spinel pol ymor ph of f or st er i t e (olivine), thought to compr

ise the earth’s inner mantle

Table I. (continued)

Spinel Compounds: Background & Historical Perspective

By

Kurt E. Sickafus and Richard Hughes

Submitted to the Journal of the American Cerami c Society, November, 1999

Others

Nigeria,

Cambodia,

Afghanistan.

There are reports of a recent find of a large deposit of fine pink spinel in the Pamir Mts., Tajikistan.

Pakistan

Karakoram Range

In marble

Reds, brownish to plum-red, lilac, violet, and blue.

Contain inclusions of green amphibole and rutile

Zircon is zirconium silicate, ZrSiO4, and is notable as a gem mineral for its high R.I., high dispersion, and subadamantine luster. Prior to advent of modern gem synthetics, colorless zircon was the stone of choice as a diamond simulant because of its diamond-like optical properties. Cubic Zirconia is not synthetic zircon but is zirconium oxide (ZrO2). Nearly all zircon contains trace quantities of U, Th, and Pb. The natural radioactive decay of uranium and thorium within zircon damages the crystal lattice, sometimes to the extent that crystals become virtually amorphous, and results in an atomic structural state that is referred to as "metamict". Such damage can render crystals darker and cloudier, and lowers the measurable optical and physical properties.

ZIRCON DEPOSITS

Zircon is present in trace quantities in nearly all intrusive igneous rocks, particularly granites, where it occurs as very small crystals. Gem zircon is nearly restricted in occurrence to pegmatites or to quartz-poor, alkalic intrusive rock (e.g. syenites). The gem gravels of Thailand are the most important commercial source, followed by production from gem gravels in Myanmar and Sri Lanka.

Gem-quality zircon is normally brown, but natural blues, oranges, greens, yellows and colorless stones are known. Color is the product of color centers and/or trace quantities of Fe. Nearly any color of the rainbow can and is produced by heat treatment of brown zircon. Heat treatment of zircon is ubiquitous.

Australia: zircon is extracted from sands. The mo st importantsource is beach

deposi ts onthe coast of Queensl and . Thes e produce 2 5 0 ,0 0 0 tons annually.

Austria, Tyrol region : zircon has been found associated with pink titanite

andchl ori te. Brazil, Minas Gerais - : has produced light green

transparentcrystal s. J Canada, Renfrew County, Ontario : crystals

up to one foot long and weighing 7kg havebeenobserved.

Germany, Eifel Mountains : as small druses of col orless zircons in trachyte cavities.

Russia, Ilmen Mountains : crystals up to 3.5k g of prismatic habit occur in veins in

granite associated with orthoclase, mica and apati te andi nthe

Ural Mountains where crystals up to 5 cm ha vebeenfound.

Thailand, Cambodia, South Vietnam : are the mai n producers of gem zircon. It

- occurs as water worn pebbles in gem gravels atdepths usually < 1 0 feet.

Sri Lankan Cut Zircon

Red Yellow

Champagne

Orange

Green

http://www.awesomegems.com/zircon.html

Yellow

Thailand (most of worlds present production), Sri Lanka, Burma, Cambodia, Laos and Vietnam. All from gem gravel deposits or gem alluvium. Produced as a byproduct of corundum mining.

Blue Zircon

"Mud Tank" Zircon Field, Harts Range,

Northwest Territory, Australia

http://www.mtlilygems.com/mineinfo/zrinfo.html

Zircon from the Harts Range,

Australia

4.14 ct., 11.3 x 8.3 x 5.9 mm, VVS

The Harts Range is a l owmountain chain ap

proximately 70 km no- rth eastof Alice Sprin

gs and extending wes t for over 100 km. The

region is arid and sup ports little vegetation

(See right) The mount ains are composed of

volcanic and strongly metamorphosed sedi

mentary rock thathas produced significanta

mounts of gold. Pegm atites scattered throu ghout the region have

been commercially mi ned for mica and bery

l. The region has also produced small amou

nts of gem quality aq uamarine, tourmaline

, garnet, and iolite.

-Approximately 150 km north northeast of Alice Springs is t

he Mud Tank gemfield (Name d for the local bore, wind pu

mp, and water storage reserv oir), famous for gem quality z ircon . At this site zircon, mas

- sive chunks of yellow green a patite, and magnetite have w

eathered from the bedrock a nd can be found by screening the thin layer of topsoil and w

ashing the gravel (See one d ay's yield above, right). The z ircon display a variety of colo

rs; brown, pink, shades of pur ple, yellow, and colorless (Se

e adjacent photo). Approxim ately 20% of the zircon from t

his particular area is suitable for faceting. Because of the h igh dispersion of zircon, this

material produces exception ally