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DIAMOND SIMULANTS, SYNTHETIC PROCESSES, AND ASSEMBLED STONES
VIII.
Diamond Simulants, Synthetic Processes, and Assembled StonesA. Terms
1. Synthetic: man-made material with essentially the same physical, chemical and optical properties as a natural gem (i.e., man-made duplicate).
2. Simulant: a material that imitates the appearance of a gem without duplicating its properties (glass, plastic, etc.).
3. Imitation: marketed as gem look-alikes.
4. Substitute: marketed as gem look-alikes.
5. Artificial: any gem material made by man.
B. Synthetic Processes
1. Flame fusion or Verneuil
a. Developed late 1800's by Verneuil for growing rubies
b. Method
1) Powdered ingredients dropped through high temperature. flame onto a rotating pedestal.
2) Pedestal slowly lowered to allow cooling and crystallization.
3) Forms a boule (carrot-shaped mass) up to nine inches in length and 1-2 inches in diameter.
4) Takes hours to days.
c. Characteristic inclusions
1) Gas bubbles.
2) Curved striae.
3) Curved color banding, color zones.
4) Unmelted powder.
d. Materials synthesized (inexpensive)
1) Synthetic corundum (star, nonphenomenal varieties).
2) Synthetic rutile.
3) Synthetic spinel.
4) Strontium titanate.
2. Flux
a. Attempts in late 1800's with commercial development successful in late 1930's and early 1940's.
b. Method
1) Powdered ingredients mixed with flux in platinum crucible.
2) May have seed crystal.
3) Flux has a lower melting point and becomes a powerful solvent for the other ingredients, causing them to melt.
4) Cooling and evaporation causes crystallization of individual crystal in clusters.
5) Most natural appearance of the synthetics.
6) Takes up to one year.
c. Characteristic Inclusions
1) Flux inclusions in wispy veils, globules, drippy icicles.
2) Platelets--platinum flakes.
3) Fingerprints--healed fractures.
4) Straight growth zoning--indicates gemstone cut from a well-developed single crystal (also occurs in natural gemstones).
5) Included seed crystals.
d. Materials Synthesized (expensive)
1) Synthetic corundum.
2) Synthetic emerald.
3) Synthetic alexandrite.
4) YAG
3. Other synthetic processes
a. Czochralski or Pulling Method
1) Developed in 1918 by Czochralski.
2) Method
a) Similar to flame fusion
b) Boule pulled upward out of a crucible with the powdered material
c) Boule is on a rotating rod with a seed crystal
3) Characteristic inclusions
a) Same as flame fusion
b) Generally flawless
4) Materials synthesized (inexpensive)
b. Hydrothermal
1) First successful hydrothermal emerald in 1960
2) Method
a) Most closely approximates nature's process with large well-formed crystals.
b) Steel autoclave used (similar to pressure-cooker)
c) Crystals grown in sealed environment with very high temperature and pressure
d) Seed plates of desired material hung in upper portion of vessel with powdered material placed in the bottom.
e) Vessel is partially filled with water and sealed with temperature and pressure increased.
f) Supersaturated solution forms, rises and deposited on seed plate.
g) Crystals 10-18 inches in length.
h) 50-80 days.
3) Characteristic inclusions
a) Cottony inclusions.
b) Conical inclusions.
c) Straight, uniform, parallel growth structures.
d) Seed plates.
4) Materials synthesized (expensive)
a) Synthetic quartz.
b) Synthetic emerald.
c) Synthetic ruby.
c. Skull melt (also hydrothermal)
1) Early 1970's in USSR producing synthetic cubic zirconia (CZ).
2) Method
a) Only method for CZ.
b) Cold crucible or skull filled with powdered ingredients.
c) Rapidly heated by a radio frequency field (similar to microwave).
d) Kept molten several hours, then skull slowly lowered.
e) Crystals grow from bottom of skull upward.
f) Approximately two inches in length.
g) 24-48 hours.
3) Characteristic inclusions-RARE
4) Material synthesized--synthetic cubic zirconia (CZ).
d. Synthetic opal
1) Suspension of uniform silica spheres by controlled mixing of chemical ingredients.
2) About one year of sedimentation of spheres into an orderly packing.
3) Consolidation, hardening and excess water removal by applied heat and pressure.
e. Ceramic technique
1) Method
a) Finely ground inorganic powder formed into solid fine-grained aggregates by heat and/or pressure.
b) Any binding agents lost during heating.
2) Materials made
a) Synthetic turquoise.
b) Imitation lapis lazuli.
c) Imitation coral.
C. Diamond Simulants
1. Distinctive Characteristics of Diamonds
a. Lack of read-through effect
1) Characteristic of various diamond simulants
2) Only used with round brilliant cut stones
a) When placed table-down on paper
b) Read print on paper through table facet
b. Natural - part of the original crystal surface seen on or near the girdle
c. Trigons on a natural = triangular etch marks are growth marks on the original crystal face.
d. Grain lines:
1) Internal graining seen on the surface in reflected light.
2) Caused by growth irregularities such as twinning.
e. Monochromatic flash
1) Synthetic cubic zirconia (CZ) generally exhibits an orange flash over most of its pavilion surface.
2) Diamond may exhibit blue or orange colors in its pavilion, but in most cases these colors extend only across one or two facets.
3) Caused by higher dispersion of diamond simulants.
f. Relationship of specific gravity to size.
1) Diamond has much lower specific gravity than most diamond simulants.
2) Measuring specific gravity.
a) Heft (rough estimate)
b) Heavy liquids (estimate)
c) Hydrostatic balance (exact)
3) Specific gravity (SG) can be related to a gemstone's size.
a) Higher the SG, the smaller the stone is per carat and vice versa.
b) Diamond with a lower SG than most simulants will be smaller per carat than simulants.
g. Immersion reflection patterns.
1) Specific gravity liquids causes different reflection patterns when diamond simulant is immersed.
2) Best with stones cut as round brilliants.
h. Fluorescence
1) Diamond generally fluoresces stronger under long wave (LW) than short wave (SW) ultraviolet (UV) light.
2) Most simulants fluoresce stronger under SW.
i. Thermal conductivity
1) Thermal test.
2) Measures change in gemstone's surface temperature.
3) Diamond has highest thermal conductivity for any gem.
4) Nearly infallible.
D. General Characteristics of Simulants
1. Possible read-through effect.
2. High dispersion
a. Flash of rainbow colors on table.
b. Monochromatic color flash on pavilion facets.
3. Higher SG than diamond.
4. Characteristic immersion reflection patterns.
5. Characteristic fluorescence.
6. Lacks thermal conductivity.
7. Doubly refractive.
a. Diamond is singly refractive (isometric crystal system).
b. Simulant may show doubling of facets.
8. Lower refractive index.
a. Causes lower luster.
b. Diamond has highest luster of any gem.
9. Lower hardness.
a. Diamond hardest gem material.
b. Causes simulants to have abraded facets, scratched surfaces.
E. Description of Simulants
1. Zircon
a. Natural gemstone of many colors.
b. DR
2. Synthetic rutile.
a. DR
b. Higher RI than diamond
c. Very strong dispersion.
1) Key indication.
2) Flash of colors.
d. Light yellow, not colorless.
e. Low hardness (abraded facets).
3. Strontium titanate.
a. Higher dispersion that diamond.
b. SR
c. Low hardness (abraded facets)
d. Most often seen in doublets.
4. YAG (yttrium aluminum garnet)
a. Not a synthetic garnet.
1) Structurally is garnet-like.
2) Chemically is not garnet.
b. Lower RI than diamond.
c. Popular simulant until replaced by CZ.
5. GGG (gadolinium gallium garnet)
a. Also garnet-like.
b. Lower RI than diamond.
c. Scratches easily.
6. Synthetic cubic zirconia (CZ)
a. Rare natural counterpart.
b. Nearly as brilliant as diamond.
c. Slightly more dispersion than diamond.
d. More durable than other simulants (Moh's hardness -81/4-81/2)
7. Doublets (described in next section)
a. Common diamond simulants.
b. Any of above described simulants as doublets.
F. Assembled Stones
1. Doublets
a. Two pieces of gem material fused together by heat (garnet and glass doublets).
b. Two pieces of gem material cemented together with a colorless cement (opal doublet).
2. Triplets
a. Two pieces of gem material cemented together with a colored cement (imparts color to the stone).
b. Three pieces of gem material cemented together with a colorless cement (opal triplet).
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