(1) 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

(2) Geological Origin

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

• pegmatites
• gem gravels

Chrysoberyl is recovered from several localities

(3) 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 SHOWING THE COLOR CHANGE PHENOMENON:

1. Corundum (containing ~20 % Cr)
2. Ruby
3. 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.

HERE ARE A COLLECTION OF CHRYSOBERYL IMAGES

(1) What is Spinel?

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

(2) Geological:

• 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.

HERE ARE SOME SPINEL IMAGES!

• 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

(2) Where is it found ?

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

VIEW SOME IMAGES OF RUTILE INCLUSIONS

Questions on the lecture

"Zircon"

"Quartz"

To gemstones section

To Course Materials

To Index Mineral Reference Glossary