Block 3 Treatments, synthetics & testing

Question 1

Define treatment

Answer 1

The process of artificially enhancing the appearance or properties of a gemstone

Question 2

What is the process in sugar-treating an opal?

Answer 2

Opal is soaked in sugar and then dropped in acid to give appearance of a black opal
To identify this treatment look for dark specks in pale matrix

Question 3

Why is impregnation and filling used as a treatment in porous stones?

Answer 3

It improves stability, colour and clarity

Question 4

What key features would you look for in a fracture filled emerald?

Answer 4

Mossy inclusion
Yellow flash
filling material may fluoresce

Question 5

What key features would you look for in a fracture filled corundum?

Answer 5

Blue flash
Bubbles in filled fracture
Iridescence in filled fractures
Change in lustre on the surface

Question 6

What year was heat treatment first used?

Answer 6

1970

Question 7

What is the temperature needed to heat treat corundum?

Answer 7

up to about 1900 degrees

Question 8

How does the amount of oxygen during heating effect the resulting colour of the stone?

Answer 8

Heat + Oxygen = Ferric Fe3+ (yellow) the atom loses an electron

Heat - Oxygen = Ferrous Fe2+ (Blue) The atom gains an electron

Question 9

If a Zircon is heated in an oxidizing atmosphere what colour will it turn?

Answer 9

White or golden yellow

Question 10

If a zircon is heated in a reducing atmosphere what colour will it turn?

Answer 10

Blue

Question 11

If a pale blue sapphire is heated in a reducing (1600-1900 degrees) atmosphere what is happening?

Answer 11

A more intense blue will be produced this is due to the interaction of iron (Fe2+) and titanium.
it will change from Ferric (yellow) to Ferrous (blue)

Question 12

What happens when you heat a dark blue stone in oxidizing (800-1900 degrees) atmosphere?

Answer 12

Changes from a Ferrous (Fe2+) to a Ferric (Fe3+) and the colour will become paler

Question 13

If rutile is released in corundum and dissolved by heating to 1750 degrees what is the effect of this?

Answer 13

More transparent stones. Helps to remove silk and stone becomes Ferrous
This is done rapidly

Question 14

What happens to titanium rich corundum that is heated at slightly lower temp (1100-1400) for 2 weeks?

Answer 14

Improves stars as rutile is titanium oxide

EXSOLVED

Question 15

What and why is surface diffusion treatment used?

Answer 15

When corundum cannot be enhanced by heating it is embedded in a chemical containing colouring elements and then heated (1600-1750). However only penetrates stone to a shallow depth.
Can be seen when stone is immersed and the colour distribution will follow the facets

Question 16

What is lattice diffusion?

Answer 16

When the material is heated in the presence of another material ie beryllium.
This treatment can penetrate much deeper than surface diffusion

Question 17

What is irradiation?

Answer 17

Bombardment with either atomic particles (diamond) or gamma rays (most other stones).
It is used to change the colour of a stone and can either be permanent or temporary

Question 18

If a greenish blue aquamarine is heat treated what colour will it go?

Answer 18

Blue - as needs higher concentration of of ferrous ions

Question 19

If greenish blue aquamarine is irradiated what colour will it go?

Answer 19

Yellow - Needs higher concentration of ferric ions

Question 20

What does irradiating a diamond do?

Answer 20

Displaces carbon atoms

Question 21

What does HPHT treatment do to diamonds

Answer 21

Heating to above 2000 degrees can alter the colour
Brown to grey type 11a (no nitrogen) become lighter in colour
Nitrogen bearing stones become yellow to to green with strong fluorescence
If they contain boron they become blue

Question 22

In synthetic production of gemstones what does cyrstallisation involve

Answer 22

Nucleation and crystal growth

Question 23

Explain what nucleation is in crystallisation

Answer 23

Occurs when atoms come together in the correct orientation

seed crystals are usually provided to take the place of nuclei

Question 24

Explain what crystal growth is

Answer 24

Atoms join the seed or nucleus to forma regular structure

Question 25

Give examples of crystallization from a melt

Answer 25

Verneuil flame-fusion
zone melting
crystal pulling
skull melting

Question 26

Explain the verneuil process

Answer 26

Powder of same composition as the gemstone falls through an oxy-hyrdogen flame (2000 degrees)
The powder melts and solidifies as a boule
Takes a couple of hours to grow about 10cm per hour

Question 27

Explain the czochralksi or crystal pulling method

Answer 27

Seed crystal brought into contact with the surface of a melt of the same composition
crystallisation happens at the end of the seed which is rotated and slowly raised
A rod shaped crystal forms as crystallization continues.
Used to make corundum, spinel, alexandrite, YAG

Question 28

Explain the process of zone melting

Answer 28

Only a part of the feed material is melted at one time
feed material may be powder or a sintered rod
Melting beings at one end and then zone melting travels along to the other end

Question 29

Explain the process of skull melting

Answer 29

A cup made of copper pipes on a pierced copper base
water flows through the tubes to keep temp below melting point of copper
a shell of unmelted zirconia powder contains the melt
The skull containing the molten zirconia is slowly lowered out of the heating coil
columnar crystals grow from the cooling base until the contents are solid

Question 30

Explain the process of flux melt

Answer 30

Flux is a molten chemical which will dissolve the chemical ingredients of a gemstone
the gemstone crystallises from this solution
Flux is held in a heat resistant crucible of platinum or graphite
temps are lower than verneuil and takes longer
Synthetic emeralds can take up to a year to produce

Question 31

Explain the hydrothermal process

Answer 31

Raw materials are enclosed in a partially water filled sealed metal container called the autoclave along with seed plates
Water is heated to 600 degrees and becomes a high pressure dense vapour capable of dissolving materials which are usually insoluble
as the water cools crystal growth occurs on the seed crystals; usually 5mm in 2 weeks