Chrysoberyl: a gemstone with many faces

withmanyfaces
a gemstone
Chrysoberyl:
i''i*'l" l.)t.li.S,.Liänni
is a mineral
thatoftenformsin pegmatite
dykesbutalso
Chrysoberyl
BeAlz0+
andmarbles.
lt crystallises
as
in metamorphic
rockssuchasmicaschists
intwinsandtrillings.
Chrysoberyl
owes
orthorhombic
crystals
andoftenappears
(Be),a lightchemical
element
thatis alsoa
itsnameto thecontent
of beryllium
beryl.
Besides
ofthiscommon
feature
thereis nolink
constituent
ofthemineral
minerals.
A greatnumber
of chrysoberylsources
liein
between
thetwodifferent
pebbles
in gravel.
Asthe
deposits
andstones
arefoundasrounded
secondary
m i n e r a l h aash i g hh a r d n e s( 8s . 5 ) a nndoc l e a v a gi tes e r v eassa d u r a b laen d
gemstone.
Thecrystal
andsimple
chemical
composition
allows
resistant
structure
andthevaluesof refractive
indices
anddensity
varyin
littleatomicsubstitution,
a s m a l l r a n goen l y . T hree f r a c t i vi ned i c easr en q 1 . 7 4 - 1 . 1a5n, dn 1 1 . 7 5 - 1 . 7 5 1 .
Figure
1 givesa survey
oncrystalshapes
andcolour
Densities
are3.70- 3.75g/cm3.
rangeof chrysoberyls.
Figure1.Roughchrysoberyl
crystalsas twinsand
trillings,anda varief of coloursandeffectsfound
with chrysoberyl.
traceelements
encountered
areFe3*,
Cr3+,
V3+
andTi4*,
substituting
Chromophore
to appear
in different
colours.
forAl3*.
Thisrangeoftracesallowschrysoberyls
(colour-giving)effect.
SnandGaarealsooftenfoundbuthavenochromophore
underdifferent
names:
Chrysoberyl
isfoundinfivevarieties
upto now.Theyappear
colourless
to yellowandbrown,
transparent
Chrysoberyl,
yellowto brownandgreenish,
translucent
cat's-eye,
Chrysoberyl
transparent
Alexandrite,
blue-green,
colourchanging,
Alexandrite
cat's-eye,
blue-green,
colourchanging,
translucent
lightblue-g
reen,transpa
rent
Vanadium-Chrysoberyl,
A termoftenused
shorter
axisof anovalstone.
(sensu
stricto)
is colourless,
but
Chrysoberyl
perfect
is "milkand
to describe
a
cat's-eye
u s u a l yl ye l l o w i sghr,e e n i st ohb r o w n
d u et o
honey"
what
means
that
half
of
thecabochon
tracesof iron.Majors0urces
arein SriLanka,
i
s
m
i
l
k
w
y
h
i
t
e
a
,
n
d
t
h
e
o
t
h
e
h
r
a
l fi sy e l l o w Zimbabwe,
Brazil,
Madagascar,
andTanzania.
(Figure
transparent
brown
and
3).
I n c l u s i ofnosu n d
a r ez i r c o n
a ,p a t i t ef i,n e
0uiterarely
we havemetchrysoberyl
star
(finger-print
fissures
type).
tubesandhealing
(four
two
lntersecting
light
stones
rays)with
Becently
we havemetheattreated
chrysoberyl
lines.Majorsources
of chrysoberyl
cat's-eyes
r e) .T h e y
o f g r e e n i sy he l l o w
c o l o u(rF i g u 2
are
in
Sri
Lanka,
Zimbabwe,Brazil,
Madagassmalldiscoid
andstrongly
reflecting
contain
car,Tanzania,
thussameasfortransparent
Figure2. Faceted
chrysoberyls
improved
by a heat
t e n s i ofni s s u r essi ,m i l at 0
r w h a ti sf o u n di n
st0nes.
treament.
Largesl
slone
is
5
ct.
stones
suchasruby,sapphire,
otherheated
In 1997browncat's-eyes
havebeen
d e m a n t oei dt c, .
reported
thatwerefoundto beradioactive.
Ghrysoberyl
cat's-eyeis alwayscutin cabogottheirchocolate
These
stones
brown
t hr el i n e
c h o nt ,h eh i g h et rh ed o m et ,h es h a r p e
colour
uponartificial
irradiation.
lt isworth
Responsible
forthechatoyancy
arethousands
mentioning
imitations
of chrysoberyl
cat'sl a n n ep
ch
l se, r p e n d i c u l a r
o fv e r yf i n ep a r a l l e
eyes.A number
of yellownatural
stones
may
t o w h i c ht h el i g h lti n ew i l la p p e aW
r . i t hl e s s e r
alsoshowchatoyancy,
ase.g.quartz,
beryl,
d e n s see to f c h a n n etlhsel i g h lti n ew i l lb el e s s
scapolite,
apatite.
Further
therehavebeen
pronounced,
andthestonem0retransparent.
imitations
infibreglass
andquartz-ulexite
Thebaseof thecabochon
mustbeoriented
doublets.
p a r a l l teol t h ec h a n n e tl sh ,a tr u na l o n g
the
Figure3. Chrysoberyl
cat's-eye
of 46 ct showing
strongchatoyancy
effect.
2010 | Volume
24,Number
3
TheAustralian
Gemmologist
I ThirdOuarter
OV
Figure4. An alexandrite
of 7.52ct fron SriLanka
with excellentcolourchange.ln thispicturethe
distinctpleochroism
is well visible,a featurethat
Figure5. Absorptionspectra
gemstones
of Cr-bearing
in
conparison.Thepositionof the
mainabsorption
movesfrom
right to left fromemeraldto
ruby.Thetransmission
left of
this maximum(emerald)or right
of thenaxinun (ruby)leaves
thenecessary
transnission
that
makesthecolourappearance.
As in alexandritethenaxinun
is at 570nm,it quasiforns
is sometinesnistakenfor colourchange.
variety
Alexandrite
isthemostvaluable
(Figure
thechrysoberyls
4).Theyowe
among
races.
t h e i cr o l o ut ro c h r o m i ut m
W ek n o wc h r o m i uamsc o l o ugri v i n g
i n g r e d i ei n e
t m e r a (l dg r e e n
a )n dr u b y{ r e d ) .
latticeCrr'givesrise
Inthechrysoberyl
crysl.al
to anabsorption
bandat 570nm.Absorption
recorded
witha spectroph0t0meter
spectra
of ruby,alexandrite
andemerald
showstrong
n ra l e x a n s i m i l a r iT
t yh. em a x i m upmo s i t i of o
andruby
driteisjustbetween
thatof emerald
( F i g u r5e) .
It isthena matterof thespectral
t hether
c o m p o s i t iooftnh ei l l u m i n alni gt hw
t h es t o n e
appeab
r sl u e - g r eoernr e d I. nc o l d
o trc o l da r t i f i c ilai gl h t ,
l i g h ts, u c ha sd a y l i g h
h .i sl i g h its
a l e x a n d r iat epsp e abrl u e - g r e T
en
i ksev i o l e tb,l u ea n d
r i c hi ns h o r t ewra v e l e n g tl h
g r e e nI n
. w a r ml i g hst u c ha si n c a n d e s clei gnht t
t .h i sl i g h its
t h es t o n easp p e ar re d d i svhi o l e T
i ksey e l l o w
a n dr e d .
r i c hi n l o n g ewr a v e l e n g tl h
T h eq u a l i toyft h i sc h a n goef c o l o ui rsn o to n l y
d e p e n d ionfgt h ea m o u notf C l b u ta l s oi t s
position
Astherearetwodifferinthelattice.
byCr,it is a
entAl sitesthatmaybeoccupied
ions
matterof thedistributi0n
of present
Cr3*
0 nt h et w os i t e sh, o wg o o dt h ec o l o ucrh a n g e
c o m e sT.h eq u a l i toyf c o l o ucrh a n gael s o
when
depends
ontheorientation
of thecrystal
s colour
c u ta so n l yt h eb - v i b r a t isohno w a
Thetermalexandrite
is attributed
to
change.
s t o n ews i t hv i s i b lceo l o ucrh a n goen l yi n m a j o r
g e mr a 0 s .
a balancebetweenthecold
coloursandthe warmcolours.
Thespectralsupplyof the
illuminantthusmakesthe
colourappearance
of the
alexandrite.
Chrysoberyls
maypossess
Crtraces
but
s t i l ln o td i s p l aayc h a n goef c o l o uSr .u c hs t o n e s
arenotcalled
alexandrites,
butchrysoberyl
isthattheevaluation
of
only.Mostimportant
t h eq u a l i toyf t h ec o l o ucrh a n giesd o n eu n d e r
warmlight
twodifferent
lighting
conditions:
l
i
g
h
t
.
a n dc o l d
R u s s i aanl e x a n d r i(tF
e isg u r6e)h a v eb e e n
treated
in anexcellent
newbook(Schmetzet
reference
2010)thatmayserveasfurlher
T h i so r i g i n a lRl yu s s i av na r i e toyf c h r y s o b e r y l ,
n a m etdo h o n o ut sr a rA l e x a n dlel ,rh a sf u r t h e r Figure6.A clusterof alexandiltefron the
thematerial
sources,
of whichmayvaryin
Malysheva
Mine,UralMountains,Bussia,
in a
acnpef ennsidprehlv
nica matrix.Thecrystalsshowcyclictwinning
SriLankan
alexandrites
areusually
spare
(trillings)Lengthof sanpleapprox.3cn.
o f i n c l u s i o nbsu,th a v eu s u a l a
l yc 0 l o ucrh a n g e
fromkhakito brown,
exceptions
aTerare.
A l e x a n d rfirt oe mZ i m b a b w( N
e o v e l lios )u s u a l l y
d a r ka n do nt h ep u r p lsei d ew, i t hl i t t l ec o l o u r
(Lake
change.
Tanzanian
N/anyara,
Tunduru)
m a t e r i iasll l g h t ei n
r t o n ea n dm a ys h o wa
to goodchange
moderate
of colourBrazilian
(Hematita)
material
is highly
saturated
and
o nt h eb l u es i d ew i t ha c h a n gt eo p u r p l e .
(0rissa)
maybequiteattractive.
Indian
stones
B u r m e sael e x a n d r iat er e
sa l s or e p o r t ebdu, t
theauthor
hasseennonesofar.
a gemstone
withmanyfaces I 68- 70
Chrysoberyl:
cat's-eye
of 7.6ct fron
Figure7.An Alexandrite
Brazil.
Henatita,
FigureB.Fluidinclusionsin an alexandritefrom
UralMountains,Bussia.Thelargerraggedinclusion
60x.
showsgasbubblein liquid C0, Magnifrcatron
Figure9. Veilsof residualflux in a synthetic
aIexandrite. Magnification 30x.
stoneis
Alexandrite
asanexpensive
produced
predestined
imitated
or
t0 be
chrysoberyls
areinthe
Synthetic
synthetrcally.
crystals.
the
1960's
as
flux-grown
tradesince
L a t epr u l l ecdr y s l afl rso mB u s s iaan dJ a p a n
markets.
The
thegemstone
havereached
is,Verneuil
synoldestimitation
of alexandrite
o p ew
d r t hv a n a d i u M
ma
. ny
t h e t i c o r u n d udm
weremisled
andbought
synthetic
tourists
r oliday
A l e x a n d rdiuar i ntgh e i h
a l e x a n d riint e
imitations
havealso
tripto Egypt.
Alexandrite
olivine
or
asCr-doped
synthetic
beenidentified
garnet
glass.
changing
rareearthdoped
Colour
foralexandrite.
hasalsobeenmistaken
L a m e l lcaor l o uzro n i nm
g a yb ep r e s e n t
pulled
butnotin Czochralskr
influx-grown,
Natural
chrysoberyls
areoftenvery
crystals.
pure.Inthelaboratory,
FTIR-spectroscopy
Trace
element
analysis
offersdiscrimination.
showsa clearpattern
byED-XFA
or LA-lCP-MS
(Ti,Ga,Zn)thatareabsent
of traceelements
Fluxgrownmaterial
onthe
insynthetic
stones.
of crucible
or
otherhandmayshowpresence
(Pt,Mo).
fluxelements
Thestonewasapparently
freeof inclus i o na
s n db e c a u soef t h ec o l o ui rt w a sh a r d
to believe
thatit wasa chrysoberyl.
Whenwe
tooka shortlookonthequalitative
chemical
byEDS-XFF
we remarked
with
constituents
thatthecolour-giving
element
astonishment
wasvanadium.
lt wasneverbefore
reported
red
thatnatura
I blue-green
chrysoberyls
colou
byV doexist.
microprobe
analysis
of blueOuantitative
(measured
greenchrysoberyl
byDr.lVichael
of 0.4
Krzemnicki,
SSEF)
contained
anaverage
andtraces
of Cr,
wt-%Vz0:and0.2wt-%FezO:
S na n dG a .
among
V-chrysoberyl
is rathera novelty
'l
varieties.
In 995theauthor
thechrysoberyl
fromthe
hasreported
a setof gemstones
deposit
intheareaof Tunduru,
secondary
Tanzania.
Among
is another
District,
Southern
fromBrazil
Songea
Alexandrite
cat's-eye
Acknowledgem ents
gemstones
prettyrareandbeautiful theenumerated
and
chrysoberyl
variety
of chrysoberyl,
t er o v i d e d
S S ES
Fw i s G
s e m m o l o g Ii cn as lt i t u p
alexandrite
werelisted.
T h e icr o l o ui rnc o l dI i g hits r a t h ebrl u i s bh u t
to thestones
shownin Figures
3, 5
access
(Figure
og
ft h em i n i nagc t i v i t i e sand10thatweresubmltted
thusthat
S i n cteh eb e g i n n i n
7) We realise
notgreenish
fortest
byclients
green
chrysoberyl
was
inthearea,lightbluish
twoeffects
arepossible:
withchrysoberyl
reports.
All otherstones
makepart0f theProf.
f
o
u
n
d
t
h
a
t
m
i
g
h
r
t
e
s
e
m
b
s
l
e
o
me
sporadically
andchange
of colour.
chatoyancy
H . AH
. ä n nGi e m s t o C
n eo l l e c t i oTnh.a n kgso
garnet.
limegreengrossular
These
stones
forthequantitative
to Dr.S.lV.Krzemnicki
canbeeasywhen
Authenticityof chrysoberyl
anddonot microprobe
occurred
in sizesupt0 somecarats
analysis
of
a V-chrysoberyl
sample.
. i l so ff l u i d a
sre
i n c l u s i oanrsep r e s e nVt e
present
anycolour
change.
Wewereverymuch
alexandrite,
someshow
frequent
in Bussian
intense R e c o m m e nrdeeadd i n g
however,
to receive
a faceted
surprised
r e) .
t w op h a sfei l l i n g(sF i g u B
'10).
(Figure
green
gemof eleven
carats
bluish
S c h m e t zKe(r2 0 1 0R)u s s i aAnl e x a n d r i t e s .
mayshowfine
Fluxgrowncrystals
Science
Publishers,
Stuttgart,
Schweizerbart
to flutd
of residual
fluxverysimilar
networks
141p.
however,
fine
contain,
veils.Thefingerprints
p o l y c r y s t a lflliunxe( F i g u r9e) .
AbouttheAuthor
Prof.Dr.H.A.Hänni
wwwgemexpert.cn
All photographs
@ H.A.Hänni
blue-green
VanadiunFigurel
0.Anoutstanding
areain
Chrysoberyl
of 11.14ct fron Tunduru
Southern
Tanzania.
j ThirdOuarter
2010 | Volume
24,Number
3
TheAustralian
Gemmologist