Gemstones: A Guide to Precious and Colored Gemstones

Gemstones

A Guide to Precious, Colored and Ornamental Gem Materials

What Is a Gemstone?

A gemstone, or gem, is a natural or laboratory-grown material valued for beauty, rarity, durability or other desirable qualities and commonly cut, polished or fashioned for jewelry and ornamentation.

Most gemstones are minerals with an ordered crystal structure, but not every gem material is a mineral. Some important gem materials are organic, such as pearl, coral and amber, while others may be rocks or aggregates composed of more than one mineral.

Gemstones can be transparent, translucent or opaque and may be faceted, carved, polished into cabochons or fashioned into beads and other decorative forms.

Precious and Semi-Precious Gemstones

Traditionally, diamond, ruby, sapphire and emerald have been described as precious gemstones, while many other gems have been called semi-precious.

Today, these terms are often used more as historical or marketing classifications than strict measures of rarity or value.

Some gemstones outside the traditional precious category can be exceptionally rare, beautiful and valuable. Fine alexandrite, Paraiba-type tourmaline, spinel, jadeite and certain opals are examples of gems that can command very high prices.

Mineral Gemstones

Most familiar gemstones are minerals.

Examples include diamond, corundum, beryl, quartz, tourmaline, garnet and topaz.

Organic Gem Materials

Some gem materials originate from living organisms.

Examples include pearl, coral, amber and certain shell materials.

Rocks and Aggregates

Some decorative gems are rocks or aggregates composed of more than one mineral.

Lapis lazuli is a familiar example of a rock used as a gem material.

How Gemstones Are Identified

Gemologists identify gemstones by examining combinations of physical, chemical and optical properties.

No single property is always sufficient. Instead, identification usually relies on a group of characteristics that together distinguish one gem material from another.

Important identifying properties can include chemical composition, crystal system, refractive index, birefringence, specific gravity, hardness, cleavage, luster, fluorescence, absorption spectra and microscopic inclusions.

Chemical Composition

A gemstone's chemical composition helps determine its basic mineral identity.

Diamond

Diamond is composed essentially of the element carbon.

Chemical symbol: C

Ruby and Sapphire

Ruby and sapphire are varieties of the mineral corundum.

Chemical formula: Al2O3

Emerald

Emerald is the green variety of the mineral beryl.

Chemical formula: Be3Al2Si6O18

Crystal System

Minerals are classified according to the symmetry and arrangement of atoms within their crystal structures.

Major crystal systems include:

• Cubic
• Hexagonal
• Trigonal
• Tetragonal
• Orthorhombic
• Monoclinic
• Triclinic

Diamond belongs to the cubic crystal system, while corundum is trigonal and beryl is hexagonal.

Crystal Habit

Crystal habit describes the characteristic external shape or form in which a mineral commonly grows.

Diamond belongs to the cubic crystal system and commonly occurs as octahedral crystals, although cubes, dodecahedral forms and other combinations can also occur.

Emerald crystals commonly form elongated hexagonal prisms, while many tourmaline crystals develop long prismatic forms.

Species and Varieties

A mineral species is defined by its chemical composition and crystal structure.

A variety is a distinctive form of a mineral species, commonly differentiated by color or another physical characteristic.

Example: Corundum

Ruby is the red gem variety of corundum.

Gem-quality corundum of other colors is generally called sapphire, with color descriptions such as blue sapphire, yellow sapphire or pink sapphire.

The Beryl Family

The mineral species beryl occurs in several important gem varieties distinguished largely by color.

Emerald

Green beryl.

Aquamarine

Blue to blue-green beryl.

Morganite

Pink to peach-colored beryl.

Heliodor

Yellow to golden beryl.

Red Beryl

Rare red variety of beryl.

Refractive Index

Refractive index measures how strongly light slows and bends as it enters a gemstone.

Because different gem materials have characteristic ranges of refractive index, this measurement is one of the most useful traditional gemological identification tests.

Birefringence

Some gemstones split entering light into two rays that travel at different speeds.

The difference between the two refractive indices is known as birefringence.

Dispersion and Fire

Dispersion is the separation of white light into its spectral colors.

In faceted gemstones, visible flashes of spectral color are commonly called fire.

Diamond is well known for its combination of high refractive index and noticeable dispersion, which contributes to its characteristic brilliance and fire.

Specific Gravity

Specific gravity compares the density of a gemstone with the density of water.

Because gem species often fall within characteristic density ranges, specific gravity can help distinguish materials that look similar.

Example: Diamond

Natural diamond has a specific gravity of approximately 3.52.

This property is one of several measurements that can help distinguish diamond from other materials.

Gemstone Hardness

Hardness describes resistance to scratching and is commonly expressed using the Mohs hardness scale.

Diamond is the hardest natural material on the Mohs scale at 10, while corundum, including ruby and sapphire, ranks 9.

Hardness should not be confused with toughness. A gemstone can be very hard but still fracture or chip if struck in a vulnerable direction.

Cleavage

Cleavage describes a mineral's tendency to split along particular crystallographic planes.

Diamond has perfect cleavage in several directions, which is why a hard blow in the wrong direction can potentially split the stone.

Toughness

Toughness describes resistance to breaking, chipping and cracking.

Toughness and hardness together help determine how suitable a gemstone is for everyday jewelry.

Gemstone Luster

Luster describes the appearance of light reflected from the surface of a gemstone.

Common descriptions include adamantine, vitreous, resinous, waxy, pearly and silky.

Diamond is famous for its adamantine luster, while many colored gemstones have a vitreous, or glass-like, luster.

Transparent

Light passes through the gem readily enough that objects may be seen clearly through it.

Translucent

Light passes through, but objects cannot be seen clearly.

Opaque

Little or no visible light passes through the material.

The Historical Term "Water"

The word "water" has historically been used in the gem trade to describe an impression of transparency, brilliance or fineness.

Terms such as "first water" were once applied to particularly fine transparent gems.

This terminology is largely descriptive and traditional rather than a modern scientific gemological grading system.

Gemstone Color

Color is one of the most visually important characteristics of many gemstones.

Gem color may result from major chemical components, trace elements, structural defects, radiation exposure or combinations of these factors.

Chromium, iron, vanadium, manganese, titanium and other elements can contribute to the color of different gem materials.

Pleochroism

Some colored gemstones appear to show different colors or different color intensities when viewed from different crystallographic directions.

This optical property is known as pleochroism.

Gem cutters may consider pleochroism when orienting rough material so that the finished gemstone displays the most desirable face-up color.

Gemstone Inclusions

Gemstones may contain crystals, fluids, gases, fractures, growth structures and other internal features known as inclusions.

Inclusions can help gemologists identify a gemstone, separate natural stones from some laboratory-grown materials, detect treatments and, in certain cases, assist in determining geographic origin.

Natural Gemstones

Natural gemstones formed through geological or biological processes without human manufacture.

Laboratory-Grown Gemstones

Laboratory-grown gemstones are manufactured by humans but can have essentially the same chemical composition, crystal structure and major physical properties as their natural counterparts.

Imitations

An imitation resembles another gemstone in appearance but does not have the same fundamental chemical and physical properties.

Gemstone Treatments

Many gemstones are treated to improve color, clarity, durability or overall appearance.

Common treatments can include:

• Heat treatment
• Fracture filling
• Oil or resin impregnation
• Irradiation
• Diffusion treatment
• Dyeing
• Surface coating

Treatment type and stability can influence care requirements and market value, so significant treatments should be disclosed.

Faceted Gemstones

Transparent gemstones are often cut with flat polished facets arranged to enhance brilliance, color and overall appearance.

Common faceted shapes include round, oval, cushion, pear, emerald, marquise and radiant styles.

Cabochon Gemstones

A cabochon has a smooth rounded or domed surface rather than numerous flat facets.

Cabochon cutting is especially useful for opaque or translucent gems and for materials displaying optical effects such as asterism, chatoyancy or play-of-color.

Asterism

Asterism produces a star-like pattern across the surface of certain cabochon-cut gems, such as star sapphire or star ruby.

Chatoyancy

Chatoyancy creates a narrow moving band of reflected light often called a cat's-eye effect.

Play-of-Color

Precious opal can display shifting flashes of spectral colors known as play-of-color.

Major Gemstone Families and Species

Gemstones occur in numerous mineral species and varieties. Some of the most familiar include:

Corundum

Ruby and sapphire.

Beryl

Emerald, aquamarine, morganite, heliodor and red beryl.

Quartz

Amethyst, citrine, rock crystal, smoky quartz and related varieties.

Garnet

A diverse group including pyrope, almandine, spessartine, grossular and andradite.

Tourmaline

A complex gem family occurring in a remarkable range of colors.

Feldspar

Includes attractive gem materials such as moonstone, sunstone and labradorite.

What Determines Gemstone Value?

Gemstone value depends on many characteristics and varies by gem species.

• Color
• Clarity and transparency
• Cut quality
• Carat weight
• Rarity
• Geographic origin when relevant
• Natural versus laboratory-grown origin
• Treatment status
• Durability
• Demand and historical importance

Color and Gemstone Quality

For many colored gemstones, color is the most important visual quality factor.

Gemologists and dealers commonly evaluate color by considering hue, tone and saturation.

The most desirable color varies from one gemstone species to another.

Gemstones and Jewelry Durability

Not every gemstone is equally suitable for every type of jewelry.

Rings are exposed to frequent impact and abrasion, so harder and tougher gemstones are generally better suited to daily ring wear.

More delicate gemstones may be better protected in earrings, pendants, brooches or occasional-wear jewelry.

Gemstone Care

Proper care depends upon the identity of the gemstone and whether it has been treated.

Some gems can tolerate ultrasonic or steam cleaning, while others may be damaged by heat, vibration, chemicals or sudden temperature changes.

When in doubt, gentle cleaning with mild soap, lukewarm water and a soft brush is generally safer than aggressive cleaning methods.

Gemological Laboratory Testing

Modern gemological laboratories combine traditional observation with advanced analytical techniques.

Standard testing can include measurements of refractive index and specific gravity, microscopic examination and observation of optical properties.

Advanced instruments may be used to determine chemical composition, detect treatments, separate natural from laboratory-grown gemstones and, for some materials, investigate geographic origin.

Gemstone Buying Tips

• Identify the gemstone species and variety before evaluating value.
• Ask whether the gemstone is natural, laboratory-grown or an imitation.
• Ask whether the gemstone has received any treatment.
• Evaluate color, transparency, clarity and cut together.
• Consider durability when choosing a gemstone for everyday jewelry.
• Obtain an independent laboratory report for important or unusually valuable gemstones.
• Compare stones under similar lighting conditions whenever possible.

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