Gemmology
Uncommon Minerals as Gemstones from Tanzania
During a field trip to East African with Werner
Spaltenstein (Chantaburi), Prof. H.A. Hänni had the
chance to visit several deposits and trading places.
Besides the commercial gemstones known to come
from this area, uncommon minerals of gemstone
quality are repeatedly found. Although physical data
can be measured easily, due to overlapping with
Fig. 3: Kornerupine from Tanzania
© H.A. Hänni, SSEF 2006
Fig. 1: Diposide rough and cut stones from Tanzania.
© H.A. Hänni, SSEF 2006
other minerals a safe identification often requires
more scientific equipment than is usually available
to traditional gemmologists. Some of the stones are Fig. 4: Musgravite from Tanzania
found in their parent rock and their formation and © H.A. Hänni, SSEF 2006
origin is well documented. Others, however, stem
from gemmy gravels, and their parent rocks were into deposits in Tanzania, Madagascar and Sri Lan-
eroded hundreds of millions of years ago. These ka. Secondary deposits may thus produce similar
gravels were shed over a large area of former East stones as pebbles of rare gem minerals in all three
Gondwana landmass. Later tectonic events have regions. In Tanzania primary deposits are produc-
broken open that treasure trove and it is now split ing gemstones and secondary deposits from the old
Gondwana treasury are also being worked. A few of
these gemstones are presented here.
Fig. 2: Chondrodite from Tanzania
© H.A. Hänni, SSEF 2006
Fig. 5: Clinohumite from Tanzania
© H.A. Hänni, SSEF 2006
page SSEF Facette No. 14, © 2007
SSEF Research:
“Paraiba” Tourmalines from Brazil and Africa
Origin determination based on LA-ICP-MS analysis of trace elements
So-called “neon-blue” copper bearing elbaite tour-
malines (Paraiba tourmalines) are highly appreci- Fig 3: LIBS spectra of investigated Cu-tourmalines show-
ated in the trade, especially when they originate ing beryllium trace concentrations. © M.S. Krzemnicki,
from the classic mining area in Brazil (Paraiba and SSEF 2006
Rio Grande del Norte). After the recent discovery of
very similar looking stones in Mozambique, origin
determination became an issue for gem laborato-
ries.
The SSEF was able to establish chemical criteria
based on trace elements such as lead, gallium, bis-
muth, and others. Our study was kindly supported
by the donation and loaning of a large number of
cut and rough stones by Chico Bank (Germany)
and Werner Spaltenstein (Thailand) and data from
the laboratory of the German Gemmological As-
sociation DGemG in Idar-Oberstein. The samples
were chemically analysed with ED-XRF, and more
sophisticated techniques such as LIBS and LA-
ICPMS.
Generally, the copper bearing elbaite tourmalines Fig 4: Diagram (Pb versus log Zn) of LA-ICPMS data
from Brazil, Nigeria, and Mozambique show quite (red: Nigeria, blue: Brazil, green: Mozambique).
a large overlapping of their chemical composi- © M.S. Krzemnicki, SSEF 2006
tion. However, careful data plotting reveals distinct
chemical differences which allow us to separate the Paraiba Triplot
origin of these tourmalines in most cases. Brazil
Bi
Fig 2: Prof. Thomas Pettke and Michael S. Krzemnicki Mozambique
10
at the Geochemistry LA-ICPMS Laboratory, University of 0
00
1
Nigeria
Berne (Switzerland). © M.S. Krzemnicki, SSEF 2006
90
09
80
08
70
07
Brazil
60 Mozambique
06
Nigeria
50
05
40
04
30
03
20
02
10
01
0
0
0
Pb 0 10 20 30 40 50 60 70 80 90 100
Ga
Fig 5: Triplot (Pb - Bi - Ga) of LA-ICPMS data
(red: Nigeria, blue: Brazil, green: Mozambique).
© M.S. Krzemnicki, SSEF 2006
page SSEF Facette No. 14, © 2007
