Step1: What is an Emerald?
Updated: Nov 10
It is strange, but thinking about what should be the first thing to discuss when approaching emeralds what comes to mind is one of the most frequent questions we receive in the emerald store, "Do you have YELLOW emeralds?”
The short answer: Emeralds are green! But this question leads us into the nature of this unusual gemstone.
Emerald's chemical description is Beryllium Aluminum Silicate. This family of minerals is known as "beryl." Beryls (Be3Al2(SiO3)6) are clear, colorless in their pure form, and can contain impurities which produce varying hues. The beryl family thus includes blue aquamarines, pink morganites, golden heliodores, red beryls, even pale green beryls . Beryls are silicates, a very common class of minerals, but emeralds represent an extremely rare example of this class. This is because emerald crystals are mostly formed in metamorphic rocks, whereas other beryls occur in pegmatites. Colombian emeralds in particular are formed by hydrothermal processes, which are even more uncommon. They derive their green hue from minute traces of chromium locked within the stone's cyclosilicate crystals, where they replace some of the aluminium atoms - the same structural magic that allows rubies their fiery redness. "With less than one per cent chromium (or vanadium), [an emerald's] crystal lattice passes the green portion of white light while absorbing the red and blue."
So there are numerous beryls, identical in composition save their impurities; and only those with the “impurity” of chromium are colored green and may be called emeralds. Don´t let yourself be confused by someone trying to sell you yellow emeralds. These are simply another of the beryl family, referred to in the emerald trade as "yellow beryl" or "heliodore", and in general much less valuable.
For those whom wants to impress: here is the chemical and physical information for emerald:
Chemical composition: Beryllium Aluminum Silicate
Chemical Formula : BE3AL2SI6O18
Hardness (Moh's Scale) 7.5-8.0
Fracture: Conchoidal vitreous
Luster: Vitreous, in polished surfaces
Specific Gravity: 2.67-2.71
Crystal System: First order hexagonal prism, tabular habit, flat well-terminated surfaces common, etching and growth marks.
Optic sign and character: U (-)
Pleochroism Varies according to body color in intensities from weak to distinct
Refractive index: 1.569- 1.575 -- Varies with origin and locality
Chemical elements responsible for color: Chromium, Vanadium, Iron
Fluorescence: UV (Short wave) : variable, from inert to very pale reddish pink UV (Long wave) : variable, from inert to medium reddish pink to red
Absorption Spectra: Typical chromium with doublet in deep red at about 680-685nm line at 635-640 wide absorption band in yellow. Varies slightly with origin.
Phenomena: Chatoyance: Rare ; "Trapiche Star" fixed six-ray star (Colombian Emerald)
Thermal conductivity; Medium low
Reaction to heat: May cause fracturing or breakage
Reaction to chemicals: Resistant to all acids except hydrofluoric acid
Electric conductivity: Beryls in general are nonconductors
Modes of formation: Hydrothermal (Colombian) and in pegmatites
Cutting Style: The ideal cut depends on the shape and color distribution of the rough material. The most common cutting style used is the rectangular or square step cut with truncated corners, known as an"emerald cut." Other cutting styles suit emeralds. It is common to find emeralds in oval shapes, pear shapes and cabochons. The cushion cut (rectangular or square step cut without truncated corners) , marquise, and round or oval cuts are less common.