Mineral. Deposita (Berl.

) 15, 81-86 (1980)

MINERALIUM
DEPOSITA
© by Springer-Verlag 1980

Modes of Occurrence and Provenance of Gemstones

of Sri Lanka
K. Dahanayake
Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka

The island of Sri Lanka is underlain primarily by Precambrian metamorphic rocks

belonging to the Amphibole, Cordierite and Pyroxene facies. Gemstones occur
mainly in the Cordierite and Pyroxene facies terrains of the Southwestern and High-
land Groups of rocks. Gem mining is carried out on hill tops, river valleys and
beds and gemstones are found associated with eluvial, residual and alluvial sedi-
ments. The dominant gemstones mined are blue sapphires, rubies and beryl. The
sources of gem type minerals are determined to be garnetiferous gneisses, cordi-
erite gneisses, marbles and pegmatites.
L'~le de Sri Lanka est formSe presque entiSrement des roches mStamorphiques de
l'~ge preeambrien. Celles-ci appariiennent aux faciSs granulitiques ~ cordierite et
pyroxSne. Les pierres pr$cieuses se trouvent dans les terrains caract~risSs par
ces facies que l'on classe dans les groupes dits "Highland" et "Southwestern".
L'exploitation des pierres pr$cieuses se far aux sommets des collines ainsi que
dans les vall&es ou dans les lits des rivi~res et une bonne partie de pierres ainsi
exploitSes sont des saphirs bleus, rubis et de b&ryls. Les min&raux qui sont
l'origine des pierres pr&eieuses proviennent des gneiss ~ grenat ou ~ cordierite,
des marbres ou des pegmatites.

INT ROD U CTION noekites), quartzite, marble, garnetif-

erous gneisses, hornblende gneiss,
The island of Sri Lanka (formerly Cey- granulites and pegmatites, (b) Vijayan
lon) is underlain principally by crystal- Complex (amphibole-granulite facies) -
line rocks of P r e c a m b r i a n age except consisting of hornblende biotite gneiss,
for s o m e isolated occurrences of Juras- migmatites and granites, (c) Southwest-
sic outcrops and the narrow northwest- ern Group (cordierite granulite facies) -
ern coastal strip formed of Upper Mio- consisting of cordierite gneiss, hyper-
cene limestones and calcareous sand- sthene granites, garnetiferous gneisses,
stones (Fig. i). The P r e c a m b r i a n of the wollastonite scapolite rocks and calc
island is sub divided into following major gneisses.
divisions (Cooray, 1978): (a) Highland The major gem fields of Sri Lanka
Group (pyroxene-granulite facies) - con- Viz. Elahera and Ratnapura both lie in
sisting of hypersthene granites (char- the Highland Group terrain (Fig. i). In

oo28-4598/8o/oola/OOSl/SOl. BO
82 K. IPahanayake

LEGEND
Limestones & Sandstones -MIOCENE
Sandstones, Stltstones & - JURA55[C
Mudstone$
Vtjayan Complex )
Highland Group ~- PREcAtdBRIAN
[~ SouthweMern Group1
~-+-'] Granltes
~ Dolerlte

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F i g . 1. M a j o r g e o l o g i c a l d i v i s i o n s of Sri L a n k a

t h e s e a r e a s m o s t of the g e m e x p l o i t a t i o n W e s t e r n G r o u p t e r r a i n . G i m - m i n i n g is
is done in the flood p l a i n s and h i l l s l o p e s v e r y r a r e i n the V i j a y a n C o m p l e x e x -
c h a r a c t e r i z e d by p a r a l l e l r i d g e and v a l - cept p e r h a p s a l o n g and a r o u n d r i v e r s
ley topography. However gem pits are w h i c h have d r a i n e d the H i g h l a n d G r o u p
a l s o found o c c a s i o n a l l y i n the South o r the South W e s t e r n G r o u p t e r r a i n s .
Model of Occurrence and Provenance of Gemstones 83

Most of the earlier works on gem- From top to bottom: 6-clayey sand 5-
stones of Sri Lanka are based on their sandy soil 4-clayey sand 3-1ateritic
mineralogy (Lacroix, 1891; Coates, 1935~ rock/soil 2-coarse unsorted gem-bear-
Wells, 1956~ Wadia and Fernando, 1945: ing sediment l-weathered garnetiferous
and Cooray and Kumarapeli, 1960) ex- gneiss/marble.
cept perhaps the interesting research by
The gem bearing sediment consists
Katz (1972) on their provenance. The
of angular rock fragments, occasional
purpose of the present paper is to brief-
rounded pebbles and cobbles, dominant
ly describe the different modes of oc-
feldspar crystals and their fragments,
currence and provenance of gemstones.
unsorted sand and gravel with finer
This work is the summary of an exten-
heavy and gem type minerals: corundum,
sive study undertaken on the major gem-
spinel, garnet, tourmaline, zircon and
mining areas of Sri Lanka.
apatite - mostly conserving the crystal
contours. Generally blue sapphires and
red rubies (corundum) are the major
Geology of Gem Pits
gem stones mined.
On the basis of field and laboratory
studies three major types of gem pits
c) Alluvial Gem Pits
are recognized (a) eluvial, (b) residual
and (c) alluvial (Fig. 2).
These gem pits are located in stream
beds and flood plains of present and
a) Eluvial Gem Pits ancient rivers. A brief description of
the stratigraphy of a representative gem
These gem pits are situated mostly on pit is as follows:
tops of hills or on their slopes of 30 to
From top to bottom: 6-clay 5-clayey
40 degrees. A brief description of the
sand and rock fragments (potential gem-
stratigraphy of a respresentative pit is
bearing sediment) 4-clay and plant re-
as follows :
mains 3-unsorted sand and gravel 2-well
From top to bottom: 3-unsorted sand, sorted gem-bearing sand and gravel l-
gravel, cobbles and boulders with sur- weathered rock (belonging to Highland
face soil 2-coarse unsorted gem-bearing Group).
sediments l-weathered rock-mostly
The gem-bearing sediment consists
garnetiferous gneiss.
of rounded pebbles and cobbles with well
The gem-bearing sediment consists sorted sands and gravels. Heavy min-
of angular fragments of rocks, unsorted erals and g e m type minerals found are
sand and finer heavy minerals - for the corundum, beryl, chrysoberyl, garnet,
most part retaining their crystallographic spinel, tourmaline, feldspar, quartz and
outlines-corundum, spinel, garnet, zircon. Occasionally, cordierite, andalu-
tourmaline and zircon lie trapped under site, apatite, kornerupine, sinhalite,
the boulders and cobbles. Generally gem taafeite and ekanite are also recognized
quality stones are blue sapphires and in the sediments. Important gemstones
red rubies. mined are blue sapphires, fancy sap-
phires and rubies (corundum), alexan-
b) Residual Gem pits drite and cat's eye (ehrysoberyl) and
yellow beryl.
These gem pits are found in the region
bordering the hill slope and located to-
wards the flood plain (at the prominent Provenance of Gemstones
break-in-slope). A brief description of
the stratigraphy of a representative pit Field observations in the Ratnapura
is as follows: area show that the eluvial gem bearing
84 K, Dahanayake

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Model of Occurrence and Provenance of Gemstones 85

sediments are found exclusively on hill lie interbedded with alluvial sediments
tops and along slopes of ridges underlain such as clay and sand or as is common
by garnetiferous gneiss. As for the on a weathered bed rock of the Highland
source of these alluvial gemstones no Group. Alluvial gem sediments are
other source other than the garnetiferous characterized by the mature nature of
gneiss could be envisaged. their grains and the variety of gem min-
erals found therein show source/s vari-
In the Elahera gem field area where
able and farther away from the site of
most of the residual sediments were
deposition. The composition of these
studied, pegmatites have not been lo-
sediments shows the oc'currence of
cated on an important scale. However
every type of gemstone mined in Sri
granites are observed in this area and
Lanka. Minerals such as beryl, chryso-
metasomatic effects on the adjacent
beryl, feldspar (moonstone) and quartz
marbles have been reported by Silva
(amethyst and citrine) could have derived
(1976). Except for the skarn type marble
from pegmatites associated with High-
so formed, the other prominent rock
land Group rocks of the area. Other min-
type of the Elahera gem field is garenet-
erals such as corundum, tourmaline,
iferous gneiss. The analysis of crushed
spinel and garnets could have derived
rock samples of both these rocks reveal
mostly from garnetiferous and cordierite
the occurrence of the following gem min-
gneisses which form the ridges and also
erals:- corundum, spinel, garnet, tour-
from marble underlying the valleys where
maline and zircon as accessories. Thus,
gem-mining is done.
for the residual gem sediments, which
in fact are lying on weathered garnet- In the alluvial pits of Vijayan Complex
iferous gneiss or skarn, the major and Southwestern Group, gem sediments
source could be one or both of these lie on any rocktype belonging to these
rocks. The more or less in situ nature divisions. So far, only alluvial gemstones
of these gemstones is vouched for by have been located in the two former ter-
their hardly unaffected crystal contours. rains. However, residual and eluvial
Furthermore the associated feldspar gemstones may be mined in Southwest-
crystals and angular rock fragments too ern Group terrain in future since a po-
indicate the immature nature and poor tential source of garnetiferous gneiss
transport of the gem sediment. The over- underlies the ridges of this area. In the
lying lateritic rocks and soil point to- Vijayan Complex, the gemstones are
wards an original garnetiferous gneiss exclusively alluvial since they are trans-
or any other Fe-bearing rock weathered oorted by rivers draining either High-
due to fluctuations of ground-water lev- land G r o u p or Southwestern Group or
el. The percolating waters may have both.
facilitated the downward movement of
heavier gemstones from the weathering
rock towards the bottom thus giving rise CONCLUSIONS
to a concentration of gemstones in the
sediment, on the less weathered bed The gemstones are found to o c c u r on
rock, The association of alluvial sedi- hill tops and slopes as eluvial deposits
ments in residual gem pits indicates
and their source is undoubtedly the under-
frequent flooding by adjacent rivers.
lying garnetiferous gneisses. Residual
The occasional presence of river pebbles
gemstones occur on the plains as well as
within the residual sediment could be
at the adjacent area between slope and
explained by these sporadic flooding
floodplain or the break-in-slope region
episodes.
associated with lateritic rock or soil.
In the Ratnapura gem field area, al- Their source is either garnetiferous
luvial gem sediments are found at depths gneiss or skarn type marble or both. As
going down to as much as 50 feet below for the alluvial gemstones they occur in
the surface and these may occasionally the riverbeds and floodplains of present
86 K. Dahanayake :Model of Occurrence and Provenance of Gemstones

and ancient rivers of the Highland Series -, Kumarapeli PS (1960) Corundum in

terrain as well as in Vijayan Complex biotite sillimanite gneiss from near
and Southwestern Group terrains and Polgahawela, Ceylon: Geol Mag 97:
their sources could be pegmatites and 480-487
garnetiferous/cordierite gneisses, Katz MB (1972) On the origin of the
marble which occur in Highland and Ratnapura-type gem deposits of
Southwestern Group terrains. Ceylon: Econ Geol 67:113-115
Lacroix A (1891) Gneissose rocks of
Salem and Ceylon: Geol Surv India
Acknowledgements. The author wishes 24:157-200
to thank Messrs A.N. Liyanage and A. P. Silva KKMW (1976) Some geological as-
Ranasinghe for assistance in the field pects of the Elahera gem field, Sri
and laboratory. The services of Miss J. Lanka, 32nd Ann Sessions Sri Lanka
Wijesekera in drafting the figures and Assoc for the Advancement of Science
Mr. Cyril Dunuhappawa in typing the Wadia DN, Fernando LJD (1945) Gems
manuscript are gratefully acknowledged. and semiprecious stones of Ceylon:
Ceylon Dept. Mineral Rec., Prof
Pap 2:13-44
Wells AJ (1956) Corundum from Ceylon.
BIBLIOGRAPHY Geol Mag 93:25-31

Coates JS 1935 The Geology of Ceylon: Received: August 31, 1979

Ceylon J Sci 19:i01-187
Cooray PG (1978) Geology of Sri Lanka
Precambrian: Proceedings-3rd Re- Dr. K. Dahanayake
gional Conference on Geology and Department of Geology
Mineral Resources of Southeast Asia, University of Peradeniya
Bangkok, Thailand, 701-710 Peradeniya, Sri Lanka