This document discusses the hardness of gemstones. It begins by introducing gemology as the science of studying natural and artificial gems. Gemstones are minerals or other materials that are cut and polished for use in jewelry. Hardness is a measure of how resistant a material is to permanent shape change from an applied force. Diamond is the hardest natural material due to the strong covalent bonding between its carbon atoms. Mohs scale ranks hardness from 1-10 based on a mineral's ability to scratch others. Hardness can be tested relatively using hardness pencils or quantitatively by measuring the depth of indentations. Direction and abrasion hardness also vary between materials. Understanding hardness is important for gem identification, cutting, and durability.

1. SEMINAR ON THE
HARDNESS OF
GEMSTONE

2. Contents
 Introduction
 Gemstones
 Hardness of gemstones
 Relative & quantitative hardness
 Moh’s scale
 Hardness testing &hardness pencils
 Directional hardness
 Abrasional hardness
 Conclusion
 References

3. Introduction
 Gemmology is the science dealing with
natural and artificial gems and gemstones
 It is considered a geoscience and a branch
of mineralogy
 Some jewelers are academically trained
gemologists and are qualified to identify and
evaluate gems

4. Gemstones
 A gemstone or gem is a piece of mineral,
which, in cut and polished form, is used to
make jewellery or other adornments
 However certain rocks or organic materials
that are not minerals (such as amber or jet),
are also used for jewellery, and are therefore
often considered to be gemstones as well
 also called a precious or semi-precious
stone, a fine gem, or jewel

5. Hardness of gemstones
 Hardness is a measure of how
resistant solid matter is to various kinds of
permanent shape change when a force is
applied
 Amber is the lightest and softest
gemstone while Diamond is the hardest

6. Hardness of gemstones are
controlled by its internal
arrangement of atoms
regularly repeating, orderly pattern

7.  Hardness depends on the nature of
bonding and cohesiveness
 Polymorphic carbon is a classic example
of hardness controlled by the nature of
bonding
 Diamond owes its superior hardness to
the homodesmic nature of bonding
 In diamond the carbon atoms are linked
with their neighbours uniformly at a
distance of 1.54 Am

8. Covalent bonding in
Diamond

9.  In graphite, the bonding length is 1.42 Am
and the nature of bonding is weak
 Thus nature of bonding has kept both
these polymorphic minerals at the extreme
end in the scale of hardness
 Minerals composed of smaller atoms/ions
are harder (eg. Diamond) than those
containing larger ions (eg. gypsum)

10. Hardness is studied in two
ways
Relative hardness
Quantitative hardness

11.  Both relative and quantitative hardness are
based on scattering of one substance over the
other
 In relative hardness, Diamond point is used to
scratch the other mineral
 In quantitative hardness, any mineral with a
known hardness is used to scratch the other
 For example, of three substances A,B & C, if A
can scratch B but not C, the hardness of A is
greater than B, but less than C & obviously C
can scratch both A & B

12.  Scratch test has to be carried out with
great caution
 Often the so called scratch mark could be
the powder of the substance that is
scratched
 So after firm scratches, surface of the
mineral has to be washed and impacted
under a lens to confirm the scratch mark

13.  For determining quantitative hardness a
diamond point pressed at known pressure
is used on the indentation tester on
smooth surface of a mineral
 Depth of scratch mark or the bite depends
on the hardness of the material

14. Moh’s scale
 Hardness depends upon the forces
holding the atoms of the mineral together.
 In 1812, a scientist, Friedrick Mohs (an
Austrian mineral expert) as a method to
identify minerals.
 He selected ten minerals and arranged
them in order so that any one mineral
could be used to scratch only minerals
which are less.
 Diamond is the- hardest natural material,
140 times harder than corundum.

16. Hardness testing & hardness
pencils
 A set of hardness pencils with their tips
embedded with the mineral of known hardness
is commercially available
 Usually the sets consists of H6, H7, H7.5, H8,
H8.5, H9 & H10
 As far as possible, this test should be avoided
for a cut stone as there are many other reliable
non-destructive confirmatory tests
 In case of faceted stone, the girdle portion can
be selected and this too, with great caution

18. Hardness testing & hardness
pencils
 Common objects used for testing
hardness are:-
 Finger nail (H2.5)
 Penknife (H5.5)
 Window glass (H5.5)
 Steel file (H6.5)

19. Finger nail Penknif
e
Window glass (H5.5)

20. Directional hardness
 Although diamond is the hardest substance
there is considerable variation in hardness
from direction to direction
 It is relatively softer along planes of a cube
and hardest parallel to octahedral faces
 Cutting and polishing is easier parallel to
crystallographic directions

21. Abrasion hardness
 By observing the amount of abrasion that
a stone has undergone during
transportation, its relative hardness can be
known
 To test the abrasive strength, various
minerals of known weight can be dumped
in a drum and tumbled for a particular
period and weighed subsequently

22. Importance of hardness
 Harder the substance better polish it takes
displaying greater lustre
 For example, diamond and corundum
appear more lustrous
 It is very difficult to polish a softer stone
despite high refractive index

23. Conclusion
 A gemstone or gem is a piece of mineral,
which, in cut and polished form, is used to
make jewelry or other adornments
 Hardness is a measure of how
resistant solid matter is to various kinds of
permanent shape change when a force is
applied
 Hardness of gemstones are controlled by
its internal arrangement of atoms
 Hardness depends on the nature of bonding
and cohesiveness

24. Conclusion
 Minerals composed of smaller atoms/ions are
harder (eg. Diamond) than those containing
larger ions (eg. gypsum)
 Hardness is studied in two ways:- relative and
quantitative hardness
 Hardness pencils are commercially available
for hardness testing of gemstones
 Finger nail, penknife, window glass & steel file
are common objects used for testing hardness
of gemstones

25. References
 Class notes by Professor KNPN
 http://en.wikipedia.org/wiki/Gemology
 http://en.wikipedia.org/wiki/hardnessofgemston
es
 R.V. Karanth(2000), Gems and Gem industry
in India, GSI Memoir 45, Pp 65-69

26. Thank
You