How To Draw Spinel Gem

Chrysoberyl, Spinel, & Rutile

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Chysoberyl
Spinel
Rutile Chrysoberyl what is chrysoberyl?
where is it found?
what are its unique characteristics?

What is chrysoberyl?

COMPOSITION	BeAl2O4
HARDNESS	8.5 (3rd hardest gem!)
CRYSTAL SYSTEM	Orthorhombic
COLOR	Yellow, green and brown
VARIETIES	Yellow-green gems
	Cats eye
	Alexandrite
IMPURITIES	Fe (produces the yellow color)
	Cr (responsible for the alexandrite effect)
SPECFIC GRAVITY	3.68-3.78
FRACTURE	Conchoidal

Remember:

chrysoberyl is NOT the same as beryl

it is among the world's rarest gems

uncut crystals are commonly found in the shape of "cyclic twins"

drawing

Chrysoberyl Images

Geological Origin

Chrysoberyl is found in Be- and Cr-rich environments (not unlike beryl)

pegmatites
gem gravels

Chrysoberyl is recovered from several localities

Unusual properties

Color change
Chrysoberyl displays the "Alexandrite Effect" where the stone looks green in daylight (rich in blue light) and red in candle light (rich in red light). (Comparision of the two together.) This effect is most obvious in thick stones, and is due to the substitution of Cr+++ for Al+++.
Brilliant green to blood red are the most desirable colors for gemstones.

Chrysoberyl can be synthesized.

Other gems which show the color change phenomenon:

Corundum (containing ~20 % Cr)

Ruby

Spinel

Chatoyant Chrysoberyl:
"Cat's Eye"

due to the presence of inclusions (needle-shaped) or tube-like cavities

needles are parallel to the base of the cabochon and perpendicular to long axis of cut gem

also called "cymophane" (Greek for waving light)

note the silky sheen and similarity to star sapphires etc.

as the stone is turned, the band of light moves, like a cat's eye (note: Tiger's eye is quartz that has replaced fibrous asbestos)

most cat's eye chrysoberyl comes from Brazil.

Spinel What is spinel?
Where is it found? What is Spinel?
spinels are oxide minerals:

COMPOSITION	[Metal][Metal]2O4
	Ex: MgAl2O4, FeAl2O4
HARDNESS	8
CRYSTAL SYSTEM	Cubic
COLOR	Red, pink, green, blue, yellow, violet...
GROWTH HABIT	Octahedral crystals
SPECIFIC GRAVITY	approx. 3.6

uncut spinels

Commonly, gem spinels are red and closely resemble rubies:

Black Prince's Ruby
Timur Ruby

Spinel Images

Where is it found?

Spinel is a metamorphic mineral

gems are often found in limestones
found in gem gravels, very often along with ruby (another source of confusion)

Geographically, you can find spinel in Burma, Sri Lanka and Thailand.
It is commonly synthesized by the Verneuil method

Rutile

What is rutile?
Where is it found?

What is Rutile?

COMPOSITION	TiO2
HARDNESS	6.5
CRYSTAL SYSTEM	Tetragonal
COLOR	Usually red-brown (due to Fe impurity)
SPECIFIC GRAVITY	4.2

rutile's natural color is usually so dark that it is rarely encountered as a cut gem
natural rutile is important in gemology because of its presence as fine inclusions, often as needle like crystals, in gems. If the needles are very fine, this results phenomena described as 'asterism' and chatoyancy.
Rutile can be synthesized as a diamond simulant
it displays exceptional fire

it is too high to be a good diamond simulant
six times the fire of a diamond

Rutile Images

Where is it found ?

in many rock types as a minor constituent
it can be found at Graves Mountain, USA