What is beryl? The gem family that produces six completely different stones

The chemistry: one formula, six colours

Beryl's formula Be₃Al₂Si₆O₁₈ describes a ring silicate in which six SiO₄ tetrahedra link into a six-membered ring, and these rings are stacked into columns parallel to the c-axis of the hexagonal crystal. The aluminium atoms sit in octahedral coordination between the rings. Trace element substitution occurs primarily at these aluminium sites (and to a lesser extent at the larger channel sites in the ring structure), and it is the nature of these substitutions that determines colour (GIA Gem Reference Guide, 2006; Klein, 2002).

The beryl family: six gem varieties from one mineral formula. Each variety is the same Be₃Al₂Si₆O₁₈ crystal with different trace element substitution at the aluminium sites. Emerald (chromium/vanadium) is the most valuable; red beryl (manganese and iron in volcanic settings) is the rarest. Source: GIA Gem Reference Guide (2006); Klein, Manual of Mineral Science (2002).

Emerald: chromium in beryl

Emerald is fully covered in the Emerald section of the Gemstone Codex. The summary for beryl context: emerald requires chromium (Cr³⁺) and/or vanadium (V³⁺) as the primary colourant. Green beryl coloured only by iron is not emerald under GIA nomenclature. The geological conditions that concentrate chromium (or vanadium) in a beryl-forming environment are specific and rare, marble-hosted or schist-hosted metamorphic environments with chromium-rich ultramafic source rocks, which is why emerald is so much more valuable than other beryl varieties despite sharing the same base chemistry (GIA; see Emerald section).

Aquamarine: iron's blue

Aquamarine is beryl coloured by Fe²⁺ (ferrous iron) in a specific configuration in the beryl channel sites. The colour ranges from pale blue to vivid blue-green depending on iron concentration and the ratio of Fe²⁺ to Fe³⁺. The finest aquamarine is a deep, pure sky blue or vivid blue with no greenish component, a colour that requires sufficient iron concentration to be saturated but not so much as to produce a greenish shift. Most commercial aquamarine shows some blue-green character; the rare pure blue stones command the highest prices (GIA Gem Reference Guide, 2006, pp. 26–29).

The largest commercially significant aquamarine deposits are in Brazil (particularly Minas Gerais state, where the Dom Pedro aquamarine, 26kg of blue beryl, the largest gemstone ever cut, was found), Pakistan (Shigar Valley and related Karakoram pegmatites), Mozambique, and Nigeria. Brazil dominates production by volume; Pakistan produces some of the finest colour material (GIA; Wise, 2016).

Heat treatment of aquamarine is standard practice and universally accepted: pale yellowish-green beryl is heated to remove the yellow component, leaving a cleaner blue. The heat treatment shifts Fe³⁺ to Fe²⁺, reducing the yellow absorption and improving the blue purity. Heated and unheated aquamarine are not differentiated commercially in the way that heated and unheated corundum is; the treatment is fully accepted and the market does not premium-price untreated stones (GIA; Wise, 2016).

Morganite: manganese's pink

Morganite is beryl coloured by Mn²⁺ (manganese) in a pale pink to peach to salmon colour range. The colour is typically light in tone, morganite rarely achieves the deep saturation of rubellite or fine pink sapphire, but the pale pink-peach of the finest morganite has its own delicate character that has built a significant market, particularly for large stones in engagement rings and statement jewellery. The low-saturation, pastel character of fine morganite suits large stones (10–50 carats) better than small ones, because the colour needs sufficient depth to be visible (GIA Gem Reference Guide, 2006, pp. 32–35; Wise, 2016).

Brazil (Minas Gerais) and Madagascar are the primary morganite sources. Afghanistan produces some fine morganite from the Nuristan pegmatites. The stone was named for J.P. Morgan, the American banker and gem collector, by George Kunz of Tiffany in 1911 (GIA).

Heliodor: Fe³⁺ yellow

Heliodor is yellow to yellow-green beryl coloured by Fe³⁺ (ferric iron). The name means "gift of the sun" in Greek. Commercial heliodor ranges from pale yellow to vivid golden yellow; the finest stones approach the canary yellow of fine yellow sapphire but without the sapphire's colour depth. Heliodor from Ukraine (the Volyn deposit) and Brazil are significant sources. Heliodor is occasionally irradiated to improve colour; GIA tests for and reports irradiation (GIA; Wise, 2016).

Red beryl: the geological outlier

Red beryl is the rarest commercially available beryl variety and one of the rarest gems in the world. It differs from all other beryls in its geological setting: while emerald, aquamarine, morganite, and heliodor all form in granitic pegmatites or metamorphic environments, red beryl forms in rhyolitic volcanic environments. The Wah Wah Mountains and the Thomas Range of Utah, USA, are the only significant sources of gem-quality red beryl. The colour is produced by manganese and iron in specific oxidation states that the volcanic environment supports (GIA Gem Reference Guide, 2006, pp. 32–37; Wise, 2016).

Red beryl is almost always small: most cut stones are under 0.5 carats, and stones above 1 carat are exceptional rarities. The colour, a vivid red to pinkish-red, is genuinely beautiful, and stones above 0.5 carats of fine colour and clean clarity command prices that can exceed rubellite and approach fine spinel on a per-carat basis. Total global gem-quality production in history is estimated at only a few thousand carats (GIA; Wise, 2016).

Goshenite: beryl without colour

Goshenite is colourless beryl, beryl without any chromophore element. It is found wherever beryl occurs and has been used historically as a diamond simulant (imperfect, given different optical properties) and as raw material for early optical lenses. Goshenite has limited commercial significance as a gem today because colourless stones compete against diamond and white sapphire; it has collector interest as the "pure" end-member of the beryl family (GIA; Wise, 2016).

Physical properties shared across the beryl family

All beryl varieties share these physical properties regardless of colour: Mohs hardness 7.5–8; indistinct cleavage (not a true cleavage plane, but the crystal has a slight tendency to fracture parallel to the base); specific gravity approximately 2.72 (lower than corundum at approximately 4.0, meaning a 1-carat beryl is physically larger than a 1-carat corundum of the same cut); refractive index 1.57–1.58 (lower than corundum at 1.76–1.77, affecting cut style optimisation); uniaxial positive optical character (GIA Gem Reference Guide, 2006).

The lower density of beryl relative to corundum is commercially relevant: buyers comparing aquamarine with sapphire at the same carat weight will find the aquamarine physically larger. This makes large-carat-weight aquamarines more accessible in visual size than equivalent sapphires, partly explaining why aquamarine is used in large statement pieces more commonly than blue sapphire (GIA; Wise, 2016).