A Photogallery of Red Emerald Mirror Modifications
As discussed in The Photogallery of Red Emerald Disturbance Modifications, the added underground pressures of pneumatolytic synthesis produce altered beryl Prisms which vastly differ from typical pegmatitic growth. Minerals must fight harder against pressure to synthesize, not only increasing stress to crystals during formation, but also inhibiting size. In addition to Disturbance Modifications, pneumatolytic pressure "pushes" or "restrains" the growth of a crystal, which may cause corners and edges to "dent". These "dents" do not appear randomly, but geometrically.
Crystals "restrained" by pressure may overcome that resistance by building symmetrical, modified structures which conform to unseen boundaries of the Mirror Plane.
A Mirror-Twin or "Dihexagon" is formed when two beryl crystals form overtop of one another, along the same C-Axis, but with their six rotational axes offset. The normal prismatic faces appear broken along their centers, with facial boundaries redefined by twelve "half-sides". If pyramids appeared at the top and bottom terminations of this already-rare Dihexagon, that specimen would have Dipyramids (As discussed in An Introduction to the Hexagonal Crystal System).
Modified edges of an Emerald specimen reveal the Dihexagonal Dipyramid Mirror Plane.
Placing the planar boundaries of beryl's Ultimate Form -- the Dihexagonal Dipyramid -- around the molecular lattice of a crystal Prism allows observation of the Mirror Plane. A beryl crystal has the option to modify its structure along these imaginary planes and still maintain symmetrical (efficient) formation.
Mirror-Modifications are structural and pyramidal alterations which result in Complex Terminations to beryl's General Form.
Mirror-Modifications are "permanent" changes reflected in and reinforced by the greater crystal structure; continued crystallization would affect these disturbances only in terms of their size, and they are not usually eliminated by overgrowth.
Mirror-Modifications alter an entire crystal face and/or multiple faces, along with termination end(s). Mirror-Modifications can be caused by crystal perfection OR crystal disruption, and may demonstrate how a General Form adapted to stress through permanent structural changes. Even a red beryl specimen with slight modification is an unusual and outstanding prize in any collection.
The following modifications to crystal form are linguistically and visually defined in this Photogallery:
Dent
Bevel
Pyramidal Bevel
Stair-Step
Ledge
Pyramidal Stairs
Stairwell
Corner
Groove
Trench
Heart
These terms are used regularly in my descriptions of red beryl specimens.
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DENT
A triangular Pyramidal Point on the termination end
of a crystalline red beryl Prism.
Dent. (Pyramidal Point) A blunted alteration at the point where two termination edges and a facial edge meet. A Dent affects three sides.
Pneumatolytic pressure applied to a point may cause the triangular Pyramidal Point to appear.
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BEVEL
A beautiful Bevel on the upper left crystal face shapes a triangular shield on the Termination End.
Bevel. (Dome or Pyramidal Edge) A blunted alteration along the edge of a single crystal face with the termination end. A Pyramidal Edge affects four sides.
A Bevel along the longer edge of two crystal faces forms a Facial Dome. This rarely-observed additional crystal face would serve as one of the "extra" sides in a Dihexagonal specimen.
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PYRAMIDAL BEVEL
Pyramidal Bevel. (Sequential Domes) Any combination of two or more connected Bevels on different rotational axes of the same geometric plane.
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STAIR-STEP
A sliver of the upper left face is unformed, creating the slightly-staggered appearance of Stair-Steps.
Stair-Step. (Inverted Bevel) A Bevel pushed inward even further along the edge between one crystal face and the termination end.
While pyramidal modifications may be relatively frequent in other beryl varieties, inverted modifications are most commonly found in Emeralds forming under pressure.
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LEDGE
Ledge. (Terminated Inverted Bevel) A Stair-Step which ceased to lengthen as a crystal continued development.
Expanding in width may continue though elongation has terminated, causing the secondary layer of crystal growth to present a more interrupted appearance than the typical Stair-Step.
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PYRAMIDAL STAIRS
Pyramidal Stairs. (Sequential Inverted Bevels) Any combination of two or more Stair-Step modifications on different rotational axes of the same geometric plane.
Pyramidal Stairs have the appearance of a wrap-around porch. If stairs circle all SIX sides of a termination end, that habit is a Full Plateau -- steps numbering fewer than six result in a Partial Plateau like the Prism above.
Plateaus are discussed in the Red Emerald Disturbance Modifications Photogallery companion to this article.
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STAIRWELL
Stairwell. (Inverted Bevels in Series) Stair-Steps passing through multiple geometric planes and terminating at progressive levels.
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CORNER
Corner. (Inverted Point) A single Stair-Step at any point.
Imagine pressure continues to be applied to a Dent until the triangular Pyramidal Point snaps inward, appearing reversed geometrically to how a point would have formed had the Prism never been modified! These "opposite-formations" truly embody the meaning of "Mirror Modification".
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GROOVE
Groove. (Inverted Edge Segment) A Corner which extends from a termination to invert an edge segment along two faces for a portion of crystal length.
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TRENCH
Trench. (Inverted Edge) A Groove which inverts the edge of two faces for the entire length of a crystal.
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HEART
Heart. (Symmetrical Inverted Edge) A Trench which inverts the full edge of a crystal while maintaining reflective symmetry on the C-Axis.
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PHENOMENAL SPECIMENS
This Phenomenal Specimen offers many interesting characteristics to study, including crystal propagation, facial domes and pyramidal points.
Phenomenal Specimens. The red beryl above was photographed at the Natural History Museum of Los Angeles County. The cluster of crystals has a sidecar which comes to a sharp edge at its lower termination; an intersecting negative crystal impression possibly caused this extreme beveling, but the disruption may have also encouraged the formation of a Dihexagonal modification...a rare Facial Dome!
The terminology presented in these Photogalleries shall assist in descriptions of breathtaking Red Emerald crystal forms we have witnessed, documented and will continue to share here!
"The beryl family produces some of the most complex and impressive specimens in the world."
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