Diploma Chapter 13 Gem Treatments Study Notes

Chapter 13: Diploma GEM TREATMENTS — STUDY NOTES

Key Concepts in Gem Treatments

  • Effects of Heating, Diffusion, and Irradiation:

    • The colors observed after heating, diffusion, and irradiation depend on various factors:

    • Present coloring-causing elements

    • The effect of treatments on these elements and the crystal structure

Heating and Diffusion

  • Process and Impacts:

    • Heating and diffusion can produce and remove color and inclusions in gemstones.

    • Caution is essential as certain inclusions may expand at a greater rate than the host material when cooled too quickly.

    • Potential Results of Rapid Cooling:

      • Fractures may occur.

      • Sometimes fractures are intentional, as seen with Amber, where stress fractures are desirable.

      • An example is crackled quartz, where stress fractures can be dyed.

    • Common Practice:

    • Heat treatments are popular on pre-formed gems (shaped/cut stones with all unwanted material removed).

Corundum Treatments (Sapphire & Ruby)

  • Factors Influencing Heating Results:

    • Atmosphere (oxidizing or reducing)

    • Temperature

    • Duration of heating

    • Additives present

Example: Blue Sapphire

  • Color Dynamics:

    • Dark blue sapphire results from an excess of Fe²⁺ (ferrous iron).

    • Heating in the presence of oxygen changes Fe²⁺ into Fe³⁺, leading to a paler blue.

    • Pale blue sapphire often contains a smaller quantity of Fe²⁺.

    • Heating without oxygen changes Fe³⁺ back to Fe²⁺, producing a darker blue.

Cooling Effects

  • Slow Cooling:

    • May promote the crystallization of rutile, potentially resulting in star sapphires.

Heat Treatment Impact

  • Stability and Color Range:

    • Heat treatment provides a range of color intensity due to variable Ti⁴⁺ content.

    • Heat treatment affects the entire body of the stone.

Diffusion Process

  • Types of Diffusion Treatments:

    • Surface Diffusion: Affects the outer layer of the gem.

    • Lattice (Bulk) Diffusion: Achieves deep penetration of elements into the gem.

Process Mechanism:

  • Heating corundum to high temperatures creates holes/vacancies in the crystal lattice.

    • These vacancies can trap diffusing elements such as Be (beryllium).

    • Trace elements, as they diffuse, can modify the color of corundum.

    • Minor shifts in balance and distribution of these elements can significantly alter the observed color.

Factors Determining Color from Diffusion:

  • The exact color produced through lattice diffusion depends on:

    • Element diffused into corundum

    • The chemical composition of the stone

    • Temperature and atmosphere during treatment

Detection of Diffusion

  • Techniques Used:

    • Immersion Test: Look for color zones relevant to the stone's shape.

    • Inclusions: Can indicate high-temperature treatment—proof of heating but not necessarily proof of lattice diffusion.

    • Presence of a large quantity of rare elements can also indicate diffusion.

    • Further laboratory testing may be necessary for confirmation.

Heating and Irradiation Mechanisms

  • Reversibility:

    • In some materials, heat treatment can be reversed through irradiation.

Effects on Beryl (Aquamarine)

  • Color Dynamics:

    • Greenish-blue aquamarine:

    • Heat-treated → more intense blue.

    • Irradiation → changes it to yellow beryl.

    • Color Relations:

    • Blue color is associated with Fe²⁺ ions.

    • Yellow color is related to Fe³⁺ ions.

Mechanisms of Color Transformation:

  • Heating causes Fe³⁺ to gain an electron, creating an increased quantity of Fe²⁺ ions, thereby intensifying the blue color.

  • Conversely, irradiation results in Fe²⁺ ions losing an electron to become Fe³⁺ ions, yielding a yellow color.

Charge Transfer in Gem Color Changes

  • Fundamentals of Charge Transfer:

    • Changes caused by heating and irradiation involve charge transfer, which entails the movement of a negatively charged electron (e⁻) between ions.

Irradiation Effects on Certain Beryls

  • Maxixe Beryl:

    • Produces a deep blue color via irradiation, which fades upon exposure to light.

    • Natural blue beryl resembling irradiated samples is termed Royal or Cobalt blue.

    • Dichroism:

    • Color variations can manifest as blue, or colorless or pale pink hues.

    • Optical Properties Explained:

    • The ordinary ray exhibits strong blue, while the extraordinary ray is equally notable in aquamarine.

Quartz Treatments

  • Amethyst Characteristics:

    • Contains iron (Fe), leading to its purple color.

    • The purple color can originate from either natural or artificial irradiation.

    • Heating amethyst yields yellow or orange hues (Citrine) and may also produce Prasiolite (green quartz).

Color Determination in Quartz:

  • Colors are determined by the proportions of Fe³⁺ and Fe²⁺ ions present within the quartz structure.

Effects of Heating and Irradiation on Quartz:

  • Further irradiation of green or yellow stones can revert them to purple hues (amethyst).

  • Gamma irradiation and heat can convert prasiolite (green) to violet-blue known as “blueberry quartz.”

Mechanism of Color Production in Quartz:

  • Complex Charge Transfer:

    • Electron vibrations occur between Fe³⁺ and Fe²⁺ ionic states, absorbing visible light across yellow, green, to blue wavelengths.

    • This results in residual red and violet light being seen, contributing to the purple hue characteristic of amethyst.

    • Heating alters the vibrational balance, yielding the yellow color characteristic of citrine.

Topaz Treatments

  • Blue Topaz Production:

    • Achieved via irradiation followed by heating processes.

Color Reversibility in Topaz:

  • The color changes for topaz can be reversed, exhibiting similar mechanisms to quartz and beryl:

    • Colorless → irradiated to brown → heated to produce blue.

    • Additionally, blue topaz can be irradiated to become brown, or heated to return back to colorless.

Commonality of Blue Topaz:

  • Blue topaz is a frequently treated gemstone in the market.

Diamond Treatments

  • Irradiation Techniques:

    • Electron or neutron bombardment is employed to create color variations in diamonds.

    • This method results in uniform and stable colors ranging from blue-green to green.

    • Identified by the GR1 absorption line at 741 nm in IR spectrophotometer readings.

Mechanism of Color Creation in Diamonds:

  • Bombardment Effects:

    • It displaces carbon atoms, creating vacancies in the diamond's structure, which leads to color formation.

Heating (Annealing):

  • Effects of Heating Above 500 °C:

    • Vacancies migrate and combine with nitrogen atoms, forming new color centers, evidenced by absorption lines around 595 nm (visible spectrum) and 1936 cm⁻¹ / 2024 cm⁻¹ (IR spectrum).

Heating Above 1000 °C:

  • Results in colors ranging from orange and yellow to brown or pink.

    • This depends largely on the presence of nitrogen aggregates and the concentration of defects present in the diamond.

High Pressure High Temperature (HPHT) Treatment

  • History and Common Usage:

    • HPHT has been practiced for over 20 years, gaining popularity since the 1990s.

    • This process creates a permanent color change in diamonds.

Conditions for HPHT Treatment:

  • Requirements:

    • High pressure: Up to 60 kilobars

    • High temperature: Up to 2000 °C

    • Short-duration processing.

Results of HPHT Treatment:

  • Alters color centers within the diamond, consequently modifying the diamond's color.

    • Originally utilized to lighten brown-to-grey type diamonds, particularly type IIa, which represent less than 1% of the world's diamonds.

Study Questions

  1. What are the three main gemstone treatments?

  2. How does heating produce or remove color in gems?

  3. Why can inclusions cause fractures during heating?

  4. What differentiates diffusion and irradiation in gemstone color treatment?

  5. What does “lattice diffusion” mean?

Beryl & Aquamarine Questions:

  • What causes the blue color in Maxixe beryl, and why does it fade?

  • How do Fe²⁺ and Fe³⁺ ions influence beryl’s color?

  • How can heat and irradiation be used to change beryl between blue and yellow?

Quartz Questions:

  • What gives amethyst its purple color?

  • What happens when amethyst is heated?

  • What is “blueberry quartz,” and how is it formed?

Corundum Questions:

  • How does heating affect the color of dark blue sapphire versus pale blue sapphire?

  • What conditions promote the formation of star sapphires?

Topaz Questions:

  • How is blue topaz produced?

  • Explain the reversibility of color in topaz, quartz, and beryl.

Diamond Questions:

  • What is GR1 absorption, and what color does it indicate?

  • How does electron bombardment create color in diamonds?

  • How does annealing above 500 °C affect diamond colors?

  • Which factors influence the final color of heated diamonds?

HPHT Diamonds:

  • What are the typical pressure and temperature conditions for HPHT?

  • What is the purpose of HPHT treatment?

  • What are “Type IIa” diamonds, and why are they used for HPHT?

Answers
1. **Three Main Gemstone Treatments:**
- Heating
- Diffusion
- Irradiation

2. **Heating and Color Changes:**
- Heating can produce or remove color in gems by altering the oxidation state or concentration of color-causing elements, such as Fe²⁺ and Fe³⁺ in corundum and beryl.

3. **Inclusions and Fractures:**
- Inclusions can cause fractures during heating because different materials may expand at different rates when heated, leading to stress and potential breaks in the gemstone.

4. **Diffusion vs. Irradiation:**
- **Diffusion** involves the introduction of color-causing elements into the gem, primarily affecting the surface or significantly penetrating (in the case of lattice diffusion), whereas **irradiation** involves exposure to high-energy radiation, changing the color by altering the electron configuration of the gem's existing elements.

5. **Lattice Diffusion:**
- Lattice diffusion refers to the process by which elements penetrate deep into the gem's crystal lattice structure, resulting in color changes.

6. **Maxixe Beryl Color Causes:**
- The blue color in Maxixe beryl is caused by irradiation and the presence of Fe²⁺ ions, and it fades upon exposure to light due to the instability of the color centers created by irradiation.

7. **Fe²⁺ and Fe³⁺ Ions in Beryl:**
- Fe²⁺ ions contribute to a blue color, while Fe³⁺ ions result in yellow. The ratio of these ions influences the final color of beryl.

8. **Heat and Irradiation in Beryl:**
- Heat treatment increases the concentration of Fe²⁺ ions, enhancing blue coloration, while irradiation can switch Fe²⁺ to Fe³⁺, leading to yellow coloration.

9. **Amethyst Color:**
- Amethyst gets its purple color from Fe impurities, which can be influenced by natural or artificial irradiation.

10. **Amethyst Heating Effects:**
- Heating amethyst typically results in yellow or orange hues (Citrine).

11. **Blueberry Quartz:**
- Blueberry quartz is formed from treating prasiolite with heat and gamma irradiation, resulting in a violet-blue stone.

12. **Sapphire Color Heating Effects:**
- Heating dark blue sapphire can result in a lighter shade by converting Fe²⁺ to Fe³⁺ and vice versa.

13. **Star Sapphire Conditions:**
- The formation of star sapphires is promoted by slow cooling of corundum, allowing for the crystallization of rutile inclusions that create asterism.

14. **Blue Topaz Production:**
- Blue topaz is produced via irradiation followed by heating processes.

15. **Color Reversibility:**
- The color in topaz and other gems can often be reversed by irradiation and heating, affecting their color centers similarly.

16. **GR1 Absorption in Diamonds:**
- GR1 absorption, indicative of color in diamonds, is observed at 741 nm, associated with blue-green coloration.

17. **Electron Bombardment in Diamonds:**
- Electron bombardment creates color in diamonds by displacing carbon atoms, leading to color changes.

18. **Annealing Effects:**
- Annealing diamonds above 500 °C helps form new color centers by combining vacancies with nitrogen atoms.

19. **Factors Influencing Diamond Color:**
- The final color of heated diamonds is influenced by the presence of nitrogen and the concentration of structural defects.

20. **HPHT Conditions:**
- HPHT treatments require high pressure (up to 60 kilobars) and high temperatures (up to 2000 °C) for effective processing.

21. **HPHT Purpose:**
- The purpose of HPHT is to create a permanent color change in diamonds, especially type IIa diamonds, which constitute less than 1% of the global diamond supply.