Classic Emerald Crystal Habits: The Prism
Like the word "Emerald", there are multiple meanings to the word "Prism".
The first meaning of "Prism" deals with the scientific study of Optics. Middle School Science introduced a crystal which could split a single beam into an entire rainbow, and all those colors originated from white light. A rock in Utah which is literally Snow White birthed an entirely new rainbow to mirror the colors of Emerald…the first of many developments to reach Fairytale proportions in the poetic history of the Red Emerald!
The Bright Side of the Sun
The second meaning of "Prism" deals with the scientific study of Geometry. An Emerald in the Rough, or an Emerald's uncut crystal form, occurs in a geometric shape of a Trapezohedron, which is also known as a Prism to mineral collectors. Gemology investigates crystal Prisms in both their geometrical AND optical senses, since crystal refraction produces optical brilliance through geometric shape.
The natural profile of a crystal depends on its mineral form. The mineral name for Emerald is beryl, which falls under the hexagonal system. Beryllium atoms bond to six oxygen with the same electrical charge to keep their atoms evenly spaced in a stable ring called a Cyclosilicate. The geometry of a hexagon limits the physical ways beryl molecules may fit together, causing similar patterns to emerge in the architectural structures of a single mineral variety.
Like sugar water condenses on a wooden stick to form rock candy, microscopic plates of beryl affix to bixbyite nucleation points floating in a magmatic solution. The hexagons begin to “stack“ one upon another, on top of and around each other, coating layer after layer until their tiny shapes combine into a much larger versions of themselves and an almost imperceptible mineral wafer materializes.
Progressive growth: Hexagons affix to bixbyite nucleation points, becoming thin wafers, then small tabulars, and finally larger prismatic towers.
Tiny plates latch on to the bigger wafer to help a Red Emerald grow. Wafers from the Thomas Range show this most plainly, as Hexagonal Stacking modifications appear on termination ends of many specimens.
When the components for specimen production are removed abruptly from the crystallization environment, construction stops immediately, allowing distortions to become visible in the mineral form. High-temperature environments cool quickly, producing small, imperfect wafers, while low-temperatures can be sustained for longer periods, allowing hexagons to stack continuously in layers, which stretch until a mineral begins preferential growth along the C-Axis. When enough wafers group together to become longer than a crystal is wide, the habit has become a "Prism".
The Anatomy of a Prism
Although prisms are found in the Thomas Range, almost all prismatic specimens have been produced in the Wah-Wah Mountains. The Ruby-Violet claims are the world's only source of gem-quality, facet-grade red beryl, but even after mining this location for a quarter-century, the largest crystal discovered weighed just 30 carat, measuring a mere 22 millimeters in height by 12 millimeters in width and depth (Gems & Gemology - 1984).
Desirable prism proportions for a beryl crystal are relative to the dimensions of an ideally faceted stone. Short tabular crystals are too shallow to cut deep pavilions for proper light return, while a beryl of a needle-like length is too long for faceting without significant loss in material and aesthetics.
While Morganite is typically tabular and elongated Aquamarine is frequently seen, Emerald prisms commonly appear in proportions ideal for an Emerald cut -- this is part of the reason that faceting style bears the Emerald name. Unlike all other beryl varieties, the Red appears in the same wide portfolio of crystal forms previously unique to Green Emerald alone -- this is another validation of the Red Emerald name. Red and Green Emerald crystals are of the same nature, because they are both produced by the same exceptionally unusual geological process.
Beryl varieties typically synthesize in pegmatites (Type I), but in extraordinary circumstances can also form in pneumatolytic environments (Type III). One of the primary differences between pegmatitic and pneumatolytic is the amount of pressure present at the time of crystal formation.
Pegmatites consist of crystal-rich granites cooled in unconstrained, low-pressure environments where mineral formation could happen with few disturbances, leaving behind fine crystals with little damage in coarse-grained rock.
High-temperature magma and gas under pressure alters rock in a process known as Pneumatolysis, a rarely-occurring natural procedure which becomes Hydrothermal at lower temperatures. Hydrothermal processes are responsible for crystallization of Green Emeralds in a mica schist.
Red beryl rarely remains attached to matrix in large sizes. The dimensions of these two crystals imply loose weights between 15 and 20 carats each.
Red Emerald synthesis took place in a rhyolite lava dome, with magma under pressure that solidified into a compact stone with crystals straining against outside force to mineralize. Nearly every red beryl specimen formed in this harsh setting shows evidence of overcoming severe natural adversity, with fractures and inclusions comparable to those observed in schist-grown Emeralds. Crystals had to fight against greater pressures underground in order to "expand" and grow, which occasionally required modifications to its typical crystal structure.
Typical crystal structure can also be called a mineral's General Form. A general form for a Crystal Class in a Crystal System is created by placing one geometric plane perpendicular to each rotational axis, arranging the planes symmetrically to enclose space. This is also the number of crystal faces which form when tiny hexagonal molecules "stack" upon one another. The general form for beryl is the Hexagonal Prism.
Any modifications to a mineral's crystal structure is a modified form or a Special Form. Most modifications are alterations to architecture determined by the Mirror Plane -- the set of facial boundaries which could symmetrically exist. Imagining these possible crystal faces as they would surround the general form reveals a mineral's Ultimate Form.
The Dihexagonal Dipyramidal Ultimate Beryl Form, over the General Form of a red beryl prism. Pyramids mirror the hexagonal hourglass core.
The Ultimate Crystal Form does not have a flat end, but instead terminates as a pyramid into a single point which "mirrors" an Emerald's hourglass color zone. Beryl been known to form phenomenally large prisms, so if nothing inhibits growth, an increase in crystal size usually appears, not a pyramid.
If a pyramid were to hypothetically appear at the top and bottom terminations of an already-rare Mirror-Twin or "Dihexagon", that specimen would have "Dipyramids", creating the Dihexagonal Dipyramidal Habit (As discussed in An Introduction to the Hexagonal Crystal System). A beryl crystal may modify its structure in any way which aligns to these unseen planes.
A specimen with even slight evidence of a mirror-plane modification is unusual and an outstanding prize for any collector.
Be sure to check the blog post next week to see a description and list of these modifications, along with an exciting photo gallery of their previously-undocumented appearances in Red Emerald!