Assessing Gems like a Gemologist:

When judging a gemstone, there are many factors to consider. This article will help you understand the key concepts to grading a colored gemstone like a professional.

Judging Color : Hue, Tone, Saturation:

Hue refers to the basic body color of the gem.

Tone refers to the lightness or darkness of the stone.

Saturation refers to the intensity or strength of the color.

Each gemstone type has an ideal or most preferred Hue that is considered the best for that species, Ruby being a pure red then slightly purplish red, Emerald being a pure green then very slightly bluish green. The hue of a gem is designated by a dominant color (Blue, Green, Yellow, Orange, Red, or Purple) and a modifying color if there is one.

Each stone also has an ideal Tone that is best for that type. Usually a medium-light to medium dark is best for most gems.

In warm colors lower saturated stones appear brownish, and in cool colors they appear grayish. So if you are seeing grays and browns the stone has lower saturation levels. If you are seeing no grays or browns you have a higher saturated stone. The higher the saturation, the more valuable the stone (all other factors being equal). The most expensive stones have a VIVID or intense level of saturation.

Brilliance, Extinction, & Windowing:

-Brilliance refers to the amount of light that reflects back to the viewer from the inside of the gemstone. It is a consequence of proper angles and cutting. This is one of the main factors that determines a gemstones quality, value, and liveliness.

-Extinction refers to the dark, nearly black, facet reflections in the stone caused by pavilion angles that do not return light back to the viewer. The light enters the gem through the table and crown and bounces off a pavilion facet and exits the gem before it can bounce back to the viewer. All stones display some amount of extinction.

-Windowing refers to an area in a gemstone where light passes straight through without reflecting back to the viewer. This is usually causes by shallow angles that cannot bounce light back and instead passes straight through the gem creating a see through area or empty patch.

Each stones brilliance grade is a combination of these 3 factors. All faceted gems display some amount of brilliance and extinction, and stones with shallow angled facets will have some windowing. These factors are directly correlated to the cutting quality of the gem. To judge a gemstone properly, hold the stone within arms length from your eyes in daylight equivalent or sunlight and move it around until you get the angle that shows the best brilliance.

Example, a stone can have 50% brilliance, 30% extinction, and 20% windowing totaling 100 percent. Or 70% brilliance and 30% extinction.

Brilliance and grading:

Excellent 75%+,

Very Good 61%-75%,

Good 41%-60%,

Fair 25%-40%,

Poor <25%

This Color-change Garnet is showing

75%+ brilliance or light return, and

<25% extinction or black spots.

Gemstone Types in regard to Clarity:

Type 1 gems: these are stones that are typically very clean when mined, and are expected to have no visible inclusions. Quartz, Aquamarine, Tanzanite, Kunzite, Blue Zircon, and Green Tourmaline are a few.

Type 2 gems: These are stones that typically show some eye visible inclusions that do not detract from the stones overall beauty. Some of the most desirable gemstones fall into this category like Ruby, Sapphire, Alexandrite, Garnet, Peridot, Spinel, and all zircons except Blue, to name a few.

Type 3 gems: These stones almost always have eye visible inclusions due to growth conditions. These include Emerald, Red Beryl, Rubellite, Watermelon Tourmaline, and Sphene as well as a few others.

So when judging a stones clarity grade, one has to consider the type of gem it is. A type 3 with eye visible inclusions would not be judged as harshly as a type 1 that had the same visible inclusions.

Effects on durability and its appearance are some of the most important things to consider when judging a gemstones clarity. Use 10x magnification to judge clarity of a gem.

Cutting factors:

Other things to look for are proportions, symmetry, and polish.

Proportions should be looked at face up (looking down at the table) and in profile (looking at its side view seeing crown, girdle, and pavilion).

Face up you should be looking for symmetry, table size and centerness, and facet meet points.

Check the table and other facets with reflected light to look for polish marks left behind from the cutting process. The table will be the most important and noticeable.

In profile you want to look for Bulging where the pavilion angle is wide and adds extra weight without increasing its face up size. This usually contributes to a stone having more extinction and less brilliance, which makes it less lively.

Girdle thickness in a gem holds the most weight, as its the widest part of the stone. So a thick or very thick girdle means paying for more weight without adding face up spread.

Phenomenal Gems:

Phenomenal gems can fall into their own category, and have unique features to judge. Opal, being the most common phenomenal gem on the commercial market, displays Play-of-Color, while others like Star Sapphires and Star Rubies display asterism. These gems will be featured in a future article all about Optical Phenomenal Gemstones.

Treatments:

Treatments and enhancements can greatly affect a gems value. While some treatments are common and accepted, others are not widely accepted and can detract significantly to a stones value. Some of the most common treatments include;

Corundum (Sapphire & Ruby): Standard Heat Treatment. A majority of these gems on the market have undergone a heating treatment to improve color and its apparent clarity. Over 95% of rubies and sapphires on the market are heated to deepen color and reduce inclusions.

Emerald: Fracture filling with oil or a resin. Due to growth conditions most emeralds will have surface reaching inclusions that get filled to improve clarity and durability. These stones are designated by the amount of filling present, F1 (Minor), F2 (Moderate), F3 (Significant). Emeralds that are not treated are rare in the market and have a Premium.

Tanzanite: Virtually all tanzanite is heat-treated to transform its natural brownish hue into its signature vibrant violet-blue. So called "unheated" tanzanites that shows trichroic colors but still has any blue or violet color have been heated naturally by earth. Without any heat they would be a brown Zoisite. Stones that have been heated by manmade methods usually lose their trichroic color and do not show brown, green, yellow, or red colors.

Bleaching: Most common in Jade and Pearls, this can make porous material more brittle, and is usually followed by an impregnating process to improve durability.

Not accepted treatments in gemstones can include the following:

Ruby : Glass Filled. These stones have many surface reaching fractures that are filled with a high lead content* glass. They have durability issues and are essentially a composite of a gem and glass and therefore hold little value to collectors.

Sapphire: Beryllium Treatment/Lattice Diffusion. These stones have been heated in the presence of elements that are not naturally in the stones and therefore the color is unnatural. In blue sapphire this treatment only penetrates a shallow layer on the outer edges of the stone and can be detected by looking for darker color that follows facet junction lines. With other fancy colored sapphires like yellows and oranges, the treatment penetrates further into the stone and can be hard to detect with standard gemological tools.

Blue Spinel: These stones can be lattice diffused with elements like Cobalt, Titanium, or Nickel to create an unnatural color. This treatment usually only penetrates a shallow layer and can be lost or compromised if ever recut. Color concentration at facet junctions may also be present.

Irradiation causes an unnatural color and is done on a variety of natural gemstones like Blue Topaz, Quartz, Tourmaline, Beryl, Diamonds, and others.

Surface coatings: Some gems may have surface coatings of metal oxides or even ink. This creates an unnatural color.

Valuation:

For insurance purposes, estate planning, or high-value sales, online calculators are not enough. You will need a professional evaluation. Aside from the gemstones quality and cut factors, origin or the place the gem was mined can be another value factor. This can be hard to prove, so it is always best to get a reputable lab report that can confirm this before paying a premium.

Written By: Roland R. Felix, Graduate Gemologist (GIA)