What is chrysoberyl?

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

Geological Origin

• pegmatites
• gem gravels

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.

uncut spinels

Commonly, gem spinels are red and closely resemble rubies:

• Black Prince's Ruby
• Timur Ruby

Where is it found?

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

Rutile

What is Rutile?

• 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

Where is it found ?

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