SYNTHESIS OF GEMS

(1) Which gems can be synthesized? - Almost all gems can be synthesized!
(2) What is a synthetic

A synthetic is a man-made material, which has the same composition and crystal structure as the natural material.

(3) What are the general ways in which gems can be synthesized ?

: From melts, from solutions, ceramic and other techniques, and at high temperature and pressure

Basic Methods

FROM MELT:

Melt same composition material - melting methods differ

Not required for students of Geology 306

(i) Verneuil Technique (Flame Fusion)
- first developed ~ 1902, -especially for ruby production , also synthetic sapphire and spinel, star rubies and star sapphires, as well as diamond simulants such as strontium titanate, and rutile.
- Method involves melting powdered material, drops of melt fall onto a growing 'boule' crystal that is lowered as it grows. The carrot-shaped 'boule' is usually several 100 carat.
  -stars formed by adding rutile powder and cooling slowly.
(ii) Czochralski Process (Crystal Pulling)
-often used to make rod-shaped single crystals -esp. for ruby, sapphire, alexandrite, GGG, YAG
- molten materials of appropriate composition
  -lower a seed crystal into the melt, rotating the seed crystal
  pull the seed out of the melt slowly, rotating
  -final crystal resembles shape of crucible
(iii) Floating Zone Method
-Developed in the 1950s for production of high-purity silicon but is also used to grow gems
- form a sintered rod of fine powder, held vertically , the rod is rotated clockwise at top, counterclockwise at bottom
  An infra red heat source moves down rod - ultimately giving single crystals
(iv) Skull Melting:
used for gems that have exceptionally high melting points or very reactive - for example, to synthesize CZ (cubic zirconia), with a melting point of ~ 2750¡ C
- "Skull' is a water-cooled container; filled with material , radio frequency generator melts central portion of the material -crystals are encased in their own powder, they grow from the bottom up

Chatham is one of several manufacturers of synthetic gems.

FROM SOLUTION:

Crystals precipitate from a saturated solution -use high temperature (and pressure) to grow more perfect gems

Details not required for students of Geology 306

(i) Hydrothermal Growth: - requires use of an 'autoclave' or 'bomb' to achieve high P, T
- The principle is to increase the solubility using solutions at high pressure and temperature. The method is analogous to gem growth in pegmatites, thus, hard to identify as synthetics
  - It is especially useful for synthesis of quartz, emerald, and some unusual beryls
  -water + seed crystals into autoclave, 100's¡C - higher temperatures around the feed material - dissolution, xalize in cooler areas
(ii) Flux growth:
Growth of ruby, sapphires, emerald, alexandrite
"Flux" is a substance that when molten will dissolve a material with a very high melting point. Flux growth is useful for synthesis of compounds with high melting points Flux is used as a molten solvent to sp eed crystallization up at lower pressures and temperatures
Flux (e.g., LiO, MoO, PbF2) mixed with powder in a crucible (e.g., Pt, Ir, Au-lined crucible) The contents are melted and cooled very slowly - crystals of high-melting compound form
pour off the still molten flux
Note: sometimes seed crystals are used)

(iii) Colloidal solution: Example: for synthesis of opal - requires production of Silica spheres of equal (tiny) size

disperse an organic-silicon compound in water and then add ammonia , let spheres settle slowly , compress gently , impregnate
dubious whether more like simulants than synthetics !!

CERAMIC TECHNIQUES:

Gilson turquoise ( grind, press, heat ceramic mixture)
coral ( calcite + coloring agent)

DIAMOND SYNTHESIS:

This is not required information for students of Geology 306

Diamond synthesis was worked out only comparatively recently. Fraudulent claims:

HANNAY, 1878 : Metallic Li, bone oil, parafin in furnace 'diamond' (but natural !)

MOISSAN: 1904: C + Fe in crucible, melting and cooling (trick by assistant!)

First real synthesis of diamonds:

-1955: GE Corp'n: high P, T synthesis

today: 100 million carats of industrial quality diamond
flawless gem-quality xals in pressure chambers
1985: Sumitomo made gem-quality diamonds (supposedly for industrial purposes)

METHOD:

use C source (xals) and seed xal and Fe catalyst
keep in pressure vessel (high temperature and pressure) for some time

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