Hardness of gemstone

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