gemmology.dev – India – Alexandrite, Diamond (Panna), and Garnet

Introduction

India's gem deposits reflect the country's diverse Precambrian and Proterozoic
geology. Significant production areas include Andhra Pradesh (chrysoberyl and
alexandrite in Eastern Ghats pegmatites), Madhya Pradesh (diamond in Panna
kimberlites), and Orissa (garnet). Kashmir sapphire, though commercially exhausted,
remains India's most famous gem locality and is covered separately.

Geological Settings

Region	Province	Geological Setting	Gems
Visakhapatnam / East Godavari	Andhra Pradesh	Acidic pegmatites in Eastern Ghats khondalites	Chrysoberyl, alexandrite
Panna District	Madhya Pradesh	Majhgawan and Hinota kimberlite pipes (~1100 Ma)	Diamond
Orissa (Odisha)	Various districts	Archaean basement; Precambrian metamorphic rocks	Pyrope and almandine garnet
Tamil Nadu	Southern India	Garnet-bearing metamorphic basement	Star garnet (almandine-pyrope)

Alexandrite – Andhra Pradesh

Eastern Ghats chrysoberyl and alexandrite:

Geological Context

Chrysoberyl (BeAl₂O₄) occurs in acidic pegmatites intruding khondalites
(metamorphic granulites) of the Eastern Ghats mobile belt
Kasipathi (1996) confirmed: "Chrysoberyl occurs in the acidic pegmatite
intrusive into the khondalite of the Eastern Ghats of north coastal Andhra Pradesh"
The pegmatite supplies Be and Al; the ultramafic country rock or metamorphic
khondalite supplies Cr for alexandrite variety formation

Properties

Colour change: Green in daylight / red in incandescent (Cr³⁺ in BeAl₂O₄)
Fluorescence: Red LWUV (Cr³⁺)
RI: 1.745–1.757 (α), biaxial positive
SG: ~3.73; Hardness: 8.5

Origin Determination Note

Indian alexandrite is noted in the trade for its colour-change effect
but fine colour change quality – the definitive balance of green-to-red
response – is generally considered less striking than fine Ural (Russian)
material
This is a generalisation based on trade consensus: the sourcing literature
does not provide a peer-reviewed comparison of Indian vs Ural quality as
a verified gemmological fact; individual stones vary

Diamond – Panna District

India's only significant primary diamond source:

Geology

Panna Diamond Belt, Madhya Pradesh: kimberlite pipes (Majhgawan and Hinota,
~1100 Ma) emplaced in the Baghain Sandstone of the Vindhyan Supergroup
The basement Bundelkhand granitoids underlie the Vindhyan sedimentary cover;
diamonds occur both in the primary kimberlite and in secondary alluvial
and conglomerate deposits derived from kimberlite erosion
Active mining by NMDC (National Mineral Development Corporation)

Koh-i-Noor Attribution – Critical Note

Various famous diamonds including the Koh-i-Noor, Regent (Pitt Diamond),
and others are traditionally attributed to the alluvial workings of the
Golconda-Panna region of central India
This attribution is TRADITIONALLY ACKNOWLEDGED in historical and gem trade
literature; it is NOT confirmed by modern gemmological peer-reviewed analysis
The Rau (2007) paper on the Panna Diamond Belt confirms the geological
framework but does not attribute any specific famous diamond to Panna
For examination purposes: state that famous historic diamonds including
the Koh-i-Noor are "traditionally attributed to" the Golconda/Panna
alluvial region of India – not that their Indian origin is a confirmed fact

Koh-i-Noor Qualification

Diamond Origin Determination Caveat

Orissa Garnet

Garnet from Orissa (Odisha):

Pyrope and almandine garnet from Proterozoic metamorphic and Archaean basement
of Odisha; artisanal production
Tamil Nadu produces 4-rayed and 6-rayed star garnet (almandine-pyrope with
rutile asterism inclusions); India is a significant source of star garnet
No diagnostic origin-determination criteria specific to Indian garnet at the
bench level; provenance documentation required