Geological Guides in Locating Semiprecious Beryl Aquamarine Minerals Shri C.S. Dwivedi, Petrologist, Dept. of Mines & Geology, Udaipur During the Course of investigation for aquarmine, heliodor, bery1 etc. in Todaraisingh area of district Tonk (Raj.) centain lithological, structural and morphological controls were established which acted as guide lines in search and exploration of semiprecious bery1 in this area. Some of the geological guides enumerated below could help investigation of these minerals on other parts of the state as well. 1. Bery1/ aquamarine mineralization is found associated with pegmatitic intrusions in schistose country rock. A distinct lithological and structural control is seen in mineralization of this semiprecious variety as Aravalli schists (Less commonly gneisses intruded by pegmatites along foliation planes concordant pegmatites) act as ideal host rocks for mineralization of semiprecious bery1. 2. Pegmatites of smaller dimensions with medium size quartz, feldspar crystals favoured bery1 mineralization. 3. Intergrowth type pegmatites with partial zoning are favored in comparison to distinctly zoned pegmatites with separate outer intermediate and border zones. 4. pegmatites with pink feldspar (microcline) are usually not found favorable sites for semiprerecious bery1 mineralization. 5. pegmatite’s with coarse bery1 crystals and largemica block do not act as good host for its mineralization. 6. Semiprecious variety of bery1 is found in pegmtites having small concentration of tourmaline. Transparent gem variety is seldom seen associated with pegmatites having rich tourmaline concentration. 7. Usually small medium dimensional pegmatites of not too course in again size having smoky quartz, light pinkish white feldspar (plagioclase) microcline perthite with subordinate small mica flakes act as ideal hunting grounds. 8. Transparent fracture less fractureless semiprecious crystals are of frequent occurrence at the contact of schist with pegmatites. Here again gem variety favours quartz in preference to feldspar in partially zoned pegmatites the core portion specially interface of quartz and feldspar but more nearer to quartz are consider preferred locales. 9. Ordinary biotite schist again is not a good host. It is the recrystallised biotite schist of dark green colour (Chlorite) in contact with small pegmatites bodies which pay rich dividends. 10. Semiprecious crystals are usually found not far from the schist (about a meter or so) and may some times be seen even enclosed in the schist itself. 11. In Todaraisingh area semiprecious bery1 is seen in pockets of highly contorted foliated compressed, recrystallised biotite chlorite schist pockets copiously injected by small medium size, medium grained blocky (Intergrowth) type of pegmatites with partial zoning emplaced along foliation planes of schist. There is a zone of felsparound central quartz core in these blocky of pagmatites which along with intergrowth type of pegmatites contains semiprecious bery1 mineralization. 12. Highly contorted, folded compressed recrtstalised batiste chlorite schist forming tight small oval packets appear ideal hunting ground and is probably due to prevention of beryllium bearing emanations from being escaped, there by concentrating ‘Be’ vapours to from colored, transparent, featureless, flawless gem variety bery1 in these pockets. The ferrous ions responsible for imparting colour to the gem variety is possibly derived from the schist components and as closer to the schist components. 13. In Todaraisingh area muscovite- microcline-per-trite-pegmatites, (Blocky) show semiprecious bery1 development at the interface of central block quartz zone surrounding microcline with less frequent occurrence in the succeouter essive zone. Petrographic studies of the area suggest that bery1 crystallization took place after the formation of microcline-perthite zone and before crystallization of quartz core. These inferences could act as favourble guides in locating small suitable blocky pegmatites which may yield semiparecoius bery1 aquamarine, heliodor and other transparent varsities if bery1.