Geology of Gemstones Final Exam

What is the chemical formula for quartz?

SiO₂

What is the primary difference between macrocrystalline quartz (e.g., amethyst) and cryptocrystalline quartz (e.g., chalcedony)?

The size of the crystals

Silica Stability: What is the correct order of the varieties of silica from the least stable to the most stable?

Opal - Chalcedony - Chert

Geode and Agate Formation: A necessary first step for geodes and agates to form is what?

The creation of a void

What is the physical structure that causes the "play-of-color" in precious opal?

A microscopic grid of silica spheres that diffract light

Overburden

The waste rock or soil overlying the mineral/gem deposit that must be removed

Lode

Primary deposit; hard rock; substantial cost in personnel & equipment

Placer

Secondary deposit; lower cost; lower risk

Grade

The measure of concentration of a gemstone deposit, expressed as carats per ton

Mining and costs: lode

Substantial $ in personnel and equipment (open-pit operations to strip overburden and tunneling deep into the Earth); conventional hard rock mining methods can result in breakage or damage to gemstones; employ labor intensive hand-work or minimum-charge blasting to prevent fracturing

Mining and costs: placer

Lower-cost, lower-risk; simpler surface mining methods (digging shallow pits or shafts, recovering material directly from river sediments); action of moving water during transport smooths rough edges – makes faceting easier; transport results in cleaner stones

Property rights: ownership can be divided into surface rights and mineral rights

Surface rights = ownership and use of the land’s visible, usable surface; right to build, far, use land for residential, agricultural, or recreational purposes

Mineral rights (or subsurface rights) = ownership of the resources found below the surface (oil, gas, coal, metals, gemstones, and other valuable minerals)

Severed rights = one party owns the surface rights, and another party owns the mineral rights

Resources

Estimated amount of minerals/gems in a deposit based on the projections of geological evidence and knowledge at a given point in time

o   Inferred = tonnage, grade, and mineral content can be estimated with a low level of confidence

o   Indicated = tonnage, densities, shape, physical characteristics, grade, and mineral content can be estimated with a reasonable level of confidence

o   Measured = tonnage, densities, shape, physical characteristics, grade, and mineral content can be estimated with a high level of confidence

Reserves

Subset of resources deemed economically viable for extraction

o   Probable: economically mineable part of an indicated and, in some circumstances, measured mineral resource

o   Proven: economically mineable part of a measured mineral resource

Physical properties of organic gemstones

Created by or derived from once-living organisms (coral, jet, amber, pearl, “fossil gemstones”), softer than many gemstones (Mohs 2-4, typically not faceted), not as valuable

Nacre (mother-of-pearl) is primarily composed of what?

The mineral and protein combination aragonite (calcium carbonate) and conchiolin

What is a cultured pearl?

A pearl that is farmed instead of naturally sourced

Amber is best described as what?

Fossilized tree resin

What is the age of amber deposits?

Millions of years old

What is jet and how does it form?

Burial of waterlogged, individual pieces of wood in organic-rich, anaerobic (oxygen-deficient) sediments; environment prevents normal decay and decay-causing bacteria, allowing wood to undergo a process of carbonization

“Hard Jet” = associated with marine environments (harder, more durable, and uniform material)

“Soft Jet” = forms in freshwater environments like swamps or lakes (softer, less dense, and more brittle)

What is fashioning?

The mechanical process involving the shaping, faceting, and polishing of a rough mineral crystal

What is cabochon?

This type of gemstone fashioning results in a smooth, rounded, often domed surface with a flat or slightly domed back

Brilliant cut

Rhomboid and triangular facets in a radial pattern

Step cut

Trapezoid or rectangular facets in concentric rows

Mixed cut

Combining brilliant and step cuts

Thermal treatment (Code H) - MOST COMMON ENHANCEMENT

Expose gemstone to high temps (200ºC - 2000ºC) for specific durations in specialized furnaces (annealing)

Induces changes that intensify or alter color, remove undesirable color zoning, improve clarity by dissolving or altering inclusions

Irradiation (Code R)

Subjecting gemstones to various forms of artificial radiation, such as electron beams or neutron beams

Change colors - often followed by heat treatment

Diffusion (Code U, BE)

Surface treatment with color-causing chemical elements during high temperature heating to penetrate the crystal structure

Surface Diffusion (Code U)

Technique creates a thin layer of color concentration near the surface of a light-colored or colorless sapphire

Beryllium Diffusion (Code BE)

Technique often applied to corundum to produce yellows, oranges, and pink-orange

Clarity Enhancement (Filling) Techniques

Use substances with refractive indices like the gem material

Visually minimize the appearance of surface-breaking fissures and improving the stone’s transparency

Oiling and Waxing (Code O/W)

Colorless, viscous oil, resin, or wax into the surface-reaching fractures

RI matching diminished the visibility of the internal fractures and inclusions

Resin and Glass Filling (Code F)

Use more durable substances such as colorless glass, plastic, or resins

Ruby fractures filled with glass, improving the gem’s transparency and color appearance

Advanced HPHT Processing (Code HP)

High-Pressure, High-Temp. treatment subjects diamonds and other gems to extreme conditions, closely replicating their formation environment

Treatment removes or lessens undesirable brownish coloration

Conditions for disclosure of enhancements/treatments

Mandatory if the treatment falls into one of 3 categories:

• Non-Permanence: If the treatment is or may not be permanent (e.g., oiling or certain coatings)

• Special Care Requirements: If the treatment requires specific care beyond routine handling (e.g., fracture-filled stones that are susceptible to chemical solvents or heat)

• Significant Effect on Value: If the enhancement significantly alters the stone’s value (either increasing salability or decreasing rarity premium, such as HPHT treatment or glass filling), disclosure is required

Gemstones other than natural: synthetic

A manufactured gemstone that has the same chemical composition and atomic crystal structure as a natural stone

Gemstones other than natural: simulant

Imitation; Cubic zirconia is an example of a manufactured stone that is both a synthetic and a simulant

What are the 4Cs (diamond grading)?

Clarity, Color, Carat Weight (implied by "weight"), and Cut

What represents the highest clarity on the clarity scale?

“Flawless”

What does the term “total weight” (carat weight) in an advertisement for a piece of gemstone jewelry mean?

There are multiple stones and that is the total weight of all the stones

Purity and karat system: what is considered pure gold?

24K gold

Why are alloys made?

Precious metals are frequently combined with other metals to make jewelry because precious metals are generally too soft on their own

What is the alloy sterling silver’s composition?

92.5% silver and 7.5% copper

What is a troy ounce?

31.1 grams; 12 troy ounces in one pound

Value Comparison (per troy ounce) between precious metals

Rhodium > Gold > Platinum > Paladium > Silver