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Lecture

EARTHSC 2M03 Lecture Notes - Pokémon Ruby And Sapphire, Gemology, Aluminium Oxide


Department
Earth Sciences
Course Code
EARTHSC 2M03
Professor
Jianping Xu

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Chapter 16 Notes
Because of the large price differential that exists between natural gems and their synthetic
counterparts, it is important that the gemmologist is not only able to identify a stone but is also able
to determine whether it is a natural or a synthetic material
Gemstone stimulants can in general be readily identified as their physical constants differ from
those of the gemstone they imitate
Synthetic versions of diamond, ruby, sapphire, emerald, quartz, and alexandrite pose a difficult
problem, as their constants are very close to those of the natural stone
Other identification procedures are taken for these stones including, growth lines, colour zoning,
and features called inclusions, all of which can be used to help separate the natural from the
synthetic gemstone
Growth Lines and Colour Zoning
Curved lines of growth and curved colour zoning are commonly seen only in coloured synthetic
Verneuil corundums and in Verneuil-produced red spinel
They are due to the intermittent fall of droplets of molten alumina onto the boule’s upper surface,
and to the greater volatility of some of the colouring oxides compared to alumina
Growth lines are quite difficult to detect in all but the Verneuil ruby, but curved colour zones which
are often broad enough to be seen with the naked eye (particularly when the stone is immersed) are
easily detected in blue Verneuil sapphires and red Verneuil spinels
As an indicator of natural origin, straight colour zoning (usually following the pattern of the lateral
crystal axes) can be seen in many natural stones, including quartz, ruby, sapphire and emerald
Twinning
A twinned crystal is one which consists of two or more individual crystals which have grown
together in a crystallographic relationship to produce a symmetrical shape
Contact Twinsoccur when the twin-halves of a crystal have grown with one half rotated through
180o to the other half
Interpenetrant Twinsconsist of two or more crystals which have been grown in proximity and
have penetrated each other with a direct relationship between their axes
Repeated or multiple twinning is a feature of some gemstones such as quartz, corundum and
chrysoberyl, and can sometimes help to distinguish them from the synthetic product
Twinning of a crystal can occur both during and after its formation, the latter being due to
subsequent deformation
Types of Inclusion
Protogenic or Pre-existing these consist of minerals which were present before the host crystal
began to form
Syngenetic or Contemporary these consist of materials which were present at the same time as
the host crystal. They can be present as trapped liquid inclusions or as liquid trapped in a fracture
which has then become sealed by the host
Epigenetic or Post-Contemporary these occur after the formation of the host crystal. They include
the recrystallization in fractures of foreign materials, the development of asterism by exsolution of
titanium dioxide as in corundum, the development of internal cleavages as in topaz and moonstone,
and irradiation damage to the crystal lattice caused by radioactive minerals in the host crystal as
occurs in some green and brown zircons
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