Greenstone: The more valuable stone

Greenstone is a common generic term for valuable, green-hued minerals and metamorphosed igneous rocks and stones, that were used in the fashioning of hardstone carvings such as jewelry, statuettes, ritual tools, and various other artefacts in early cultures. Greenstone artefacts may be made of greenschist, chlorastrolite, serpentine, omphacite, chrysoprase, olivine, nephrite, chloromelanite among other green-hued minerals. The term also includes jade and jadeite, although these are perhaps more frequently identified by these latter terms.The greenish hue of these rocks generally derives from the presence of minerals such as chlorite, hornblende, or epidote.

Greenstone minerals were presumably selected for their color rather than their chemical composition. In archaeology therefore, having a loosely applied general term is at least partially influenced by the observation that ancient cultures often used and considered these various green-hued materials as interchangeable.Greenstone objects are often found very considerable distances from the source of the rock, indicating early travel or trading networks. A polished jadeite axe head in the British Museum (4000-2000 BCE) was found in Canterbury, Kent but uses stone from the Alps of Northern Italy, and objects from other parts of the world had travelled comparable distances to their findspots.

Ancient China and Mesoamerica are particularly noted for the prevalence and significance of greenstone (particularly jade) usage. Greenstones also figure prominently in the indigenous cultures of southeastern Australia, and among the Māori of New Zealand (known as Pounamu). Neolithic Europe also used greenstone, especially for prestige versions of axe tools, not made for use; comparable jade versions of tools and weapons are also found in the Olmec and other Pre-Columbian cultures and early Chinese civilization.

Emerald

Emerald is a variety of the mineral beryl (Be₃Al₂(SiO₃)₆) colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5–8 on the 10 point Mohs scale of mineral hardness.^[2] Most emeralds are highly included, so their toughness (resistance to breakage) is classified as generally poor.

Emeralds, like all colored gemstones, are graded using four basic parameters–the four Cs of Connoisseurship: Color, Cut, Clarity and Crystal. The last C, crystal, is simply a synonym for transparency, or what gemologists call diaphaneity. Before the 20th century, jewelers used the term water, as in "a gem of the finest water," to express the combination of two qualities: color and crystal. Normally, in the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emeralds, crystal is considered a close second. Both are necessary conditions. A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency to be considered a top gem.

In the 1960s, the American jewelry industry changed the definition of 'emerald' to include the green vanadium-bearing beryl as emerald. As a result, vanadium emeralds purchased as emeralds in the United States are not recognized as such in the UK and Europe. In America, the distinction between traditional emeralds and the new vanadium kind is often reflected in the use of terms such as 'Colombian Emerald

Color

Scientifically speaking, color is divided into three components: hue, saturation and tone. Emeralds occur in hues ranging from yellow-green to blue-green, with the primary hue necessarily being green. Yellow and blue are the normal secondary hues found in emeralds. Only gems that are medium to dark in tone are considered emerald; light-toned gems are known instead by the species name green beryl. The finest emerald are approximately 75% tone on a scale where 0% tone would be colorless and 100% would be opaque black. In addition, a fine stone should be well saturated, the hue of an emerald should be bright (vivid). Gray is the normal saturation modifier or mask found in emerald; a grayish-green hue is a dull green hue.
Emeralds are green by definition (the name is derived from the Greek word 'smaragdus', meaning green).Emeralds are the green variety of beryl, a mineral which comes in many other colors that are sometimes also used as gems, such as blue aquamarine, yellow heliodor, pink morganite and colorless goshenite

Treatments

Most emeralds are oiled as part of the post lapidary process, in order to improve their clarity. Cedar oil, having a similar refractive index, is often used in this generally accepted practice. Other liquids, including synthetic oils and polymers with refractive indexes close to that of emerald such as Opticon are also used. The U.S. Federal Trade Commission requires the disclosure of this treatment when a treated emerald is sold.^[10] The use of oil is traditional and largely accepted by the gem trade. Other treatments, for example the use of green-tinted oil, are not acceptable in the trade. The laboratory community has recently standardized the language for grading the clarity of emeralds. Gems are graded on a four step scale; none, minor, moderate and highly enhanced. Note that these categories reflect levels of enhancement not clarity. A gem graded none on the enhancement scale may still exhibit visible inclusions. Laboratories tend to apply these criteria differently. Some gem labs consider the mere presence of oil or polymers to constitute enhancement. Others may ignore traces of oil if the presence of the material does not materially improve the look of the gemstone.

Given that the vast majority of all emeralds are treated as described above, and the fact that two stones that appear visually similar may actually be quite far apart in treatment level and therefore in value, a consumer considering a purchase of an expensive emerald is well advised to insist upon a treatment report from a reputable gemological laboratory. All other factors being equal, a high quality emerald with an enhancement level of moderate should cost half an identical stone graded none.