Rubies

Han Htun, Myanmar Gems Laboratory, c/o No. 7, Myathitsa Road, 13 Qtr., Yankin T.S., Yangon, Myanmar and George E. Harlow, Dept. Earth and Planetary Sciences, American Museum of Natural History, New York, NY, US

 Summary:

There are two major sources of rubies in Myanmar (Burma)—Mogok (Mandalay Division) and Mongshu (southern Shan State)—and several minor sources Nawarat/Pyinlon (near Namkhan, Shan State); Tanai and Nayaseik (Kachin State); Katpana (Kachin State); and Sagyin and Yatkanzin stone tracts (Madaya township) near Mandalay.

Mogok and the smaller deposits are similarly hosted in white marble with considerable diversity among the rubies from each tract and strong similarities among the rubies between the tracts. Mongshu, although associated with metasediments and marbles, yields distinctly different rough and treated stones. Thus, stones from Mongshu are easily distinguished from those found at Mogok and the smaller deposits. These differences can be discerned using an optical microscope. Basic characteristics of stones from the minor sources are described, however further research is required to assess criteria for distinguishing among these sources and Mogok.

Methodology: Several hundred rubies from these source areas in Myanmar were examined for features that could be recorded photographically using a Mark 6 Gemolite with a 10X eyepiece normally at maximum magnification (~60x). Images were recorded on 35 mm Ektachrome tungsten film (ASA 160) with an "eye-piece" camera and printed electronically on a FUJIX Pictography 3000 (TDT process printer). Images are presented here at relatively low resolution. 

Mogok Rubies

The classic rubies from Mogok characteristically contain short, fine, rutile needles -- so-called silk. These dust-like inclusions occur either as overall cloudiness, in patches, or in distinct zonings. Generally, long slender rutile needles in reticulated patterns are more common in pinkish red rubies. Boehmite needles are restricted to twin planes and other lamella in ruby.

The other most common inclusions in Mogok rubies include colorless calcite and dolomite (sometimes yellowish)-both manifest internal twinning and/or cleave planes- prismatic apatite, feldspar, pyrrhotite, titanite, magnetite, micas, spinel, and possibly tourmaline, diopside and many other minerals. Fluid inclusions on healed fractures (healing feathers) are also typical features in Mogok rubies. Negative crystals of mixed solid and fluid are not uncommon.

Mogok rubies are known for their intense "pigeon-blood" color, enhanced by daylight fluorescence, but saturation ranges to pink. Pronounced crystallographic color zoning is rare, but subtle banding and small "swirls" of color zoning, known as treacle, are common.

Mogok-1: A combination of subhedral and irregular carbonate (calcite?) inclusions are in the center of the image; middle left inclusion shows rhombohedral cleavage. A patch of fine silk to right and fine fluid inclusions on a healed feather. [Frame is 3 mm wide.]

Mogok-2: Same stone as Mogok-1. The central inclusion shows several cleavage plains, indicating it is probably calcite. [Frame is 3 mm wide.]

Mogok-3: Several inclusions showing negative crystal form; perhaps mixed carbonate & fluid. Large ruptured/fractured mixed inclusions below and out-of-focus feather above. [Frame is 3 mm wide.]

Mogok-4: Fine oriented needles of rutile merge into denser silk to left. [Frame is 3 mm wide.]

Mogok-5: Three orientations of rutile needles (therefore, looking down c-axis) with one set forming bands of characteristic silk. [Frame is 3 mm wide.]

Nawarat Rubies

Rubies from Pyinlon on the Nawarat stone tract occur in a crystalline marble as at Mogok. Inclusion minerals in these rubies are dominated by calcite, dolomite, and rutile with lesser numbers of apatite, feldspar, and probably pyrrhotite and spinel. Inclusion morphologies vary from euhedral to rounded and irregular. Silk tends to be less pronounced (thinner or dusty) than in Mogok rubies. Silk banding is not uncommon, but color banding, if present, is not distinct. Color swirls (treacle) has been observed. The Crown of Nawarat is 5.45 carat ruby with an exceptionally fine pigeon's blood red color, however, the majority of the rubies have medium choice color with a light purplish overtone. 

Nawarat-1: Medium- to large-sized transparent inclusions (probably calcite) dominate this stone. Oriented short fibers (rutile?) coat a healed surface to upper left and a patch of silk is just below-right of center. [Frame is 3 mm wide.]

Nawarat-2: Large, irregular, transparent cleaved inclusions are probably calcite. A patch of oriented rutile silk appears at upper right. [Frame is 3 mm wide.]

Nawarat-3: Dense zone of irregular, rounded, transparent inclusions with adjacent thin veils or oriented rutile silk. [Frame is 3 mm wide.]

Nayaseik Rubies

The Nayaseik and Tanai ruby deposits are in the Kachin State in northern Burma. Stones rarely have silk or feathers and are usually very transparent. Inclusions are common, dominated by calcite, dolomite, apatite, and micas as euhedral to subhedral grains with distinct faces. Very small distinct rutile grains have been observed. Typical colors are pinkish red; very few of the prized deep-red rubies have been found.

Nayaseik-1: Subhedral to rounded inclusions of carbonate (calcite?) in pinkish stones with thin, patchy silk are typical for the locality [Frame is 3 mm wide.]

Nayaseik-2: Several large subhedral inclusions of calcite(?) are visible; lower one with reflecting crystal faces indicate a carbonate, probably calcite. Minute patches of thin silk appear at left. [Frame is 3 mm wide.]

Nayaseik-3: An unusual stone with a broad patch of silk and a large rounded inclusion of calcite(?) [Frame is 3 mm wide.]

Sagyin Rubies

Rubies were mined at the Sagyin stone tract from colluvial deposits weathered from marble during the reign of King Thibaw (1878-1886). Color varies from pinkish- to medium-red, although a few stones of intense deep red color have been reported. Stones are relatively free from silk but may contain dustings of rutile particles and transparent calcite inclusions of subhedral to rounded form. The ruby crystals are tabular prisms with triangular growth hillocks on the pinacoids.

Sagyin-1: Rounded inclusions at center are surrounded by very fine and thin oriented silk. Euhedral inclusions to lower right are a single inclusion duplicated by reflections off facets. [Frame is 3 mm wide.]

Sagyin-2: Rounded inclusions of calcite in pinkish stones with minor or no silk are typical. Source of turbidity is a fracture below that is out of focus. [Frame is ~2 mm wide.]

Katpana

Katpana rubies range from purple-pink to pinkish red, less desirable colors in Myanmar. Dense zones of silk are common, yielding many fine star rubies, so the material is mostly cut as cabochons or, alternatively, as small stones for earrings. Sorry, no photographs.

Heat-treated Mongshu Rubies

Mongshu rubies are characterized by a central blue-to-violet core in intense red crystals, healing feathers, several distinct growth zoning patterns and very fine white needles on inclusions in untreated stones (Peretti et al. 1995). Transparent calcite inclusions and dense silk zones occur less commonly than in Mogok rubies. Boehmite occurs occasionally along twin planes.

Nearly all Mongshu material is heat-treated to remove the bluish or violet tint and thereby improve the body color. Local treatments (in Myanmar) involve temperatures from 700 to 1200 degrees C, probably without flux, that is inadequate to produce flux-glass feathers in cracks. Inadequate treatment may leave zones of blue intact. Wispy, veil-like fluid-inclusion feathers or "fingerprints", exsolved rutile needles and fine particles resembling snowflakes are diagnostic features in the treated stones. Comet-shaped silk stringers, as observed in Kashan synthetic rubies, have been detected in treated Mongshu rubies. Exsolved rutile grains and whitish healing feathers produce a hazy or "sleepy" appearance in cut stones. Inclusions surround by discoid stress feathers and inclusions that are whitish or bubble-like with high-relief rims are common in heat treated stones.

Mongshu-1: Stress fracture "feather" (upper left) and a healed fracture decorated with trains of irregular fluid inclusions ("fingerprint") are typical of heat-treated Mongshu stones. These Myanmar treated stones lack the glassy features of the high-temperature Thai-treated equivalents. [Frame is 3 mm wide.]

Mongshu-2: A single, healed stress fracture decorated with trains of irregular fluid inclusions, typical of treated Mongshu stones. [Frame is 3 mm wide.]

References:

• Hughes, R.W. (1997) Burma (Myanmar) 300-343. In Ruby & Sapphire. RWH Publishing, Boulder, CO. pp. 511.
• Kammerling, R.C., and Scarratt, K. (1994) Myanmar and its gems--An update. Journal of Gemmology 24, 3-40.
• Peretti, A., Schmetzer, K., Bernhardt, H.-J., and Mouawad, F. (1995) Rubies from Mong Hsu. Gems and Gemology 31, 2-26.
• Sanchez, J.L., Osipowicz, T., Tang, S.M., Tay, T.S., and Win, T.T. (1997) Micro-PIXE analysis of trace-element concentrations of natural rubies from different locations in Myanmar. Nuclear Instruments and Methods in Physics, B 130, 682-686.
• Smith, C.P., and Surdez, N. (1994) The Mong Hsu ruby: a new type of Burmese ruby. JewelSiam 4, 82-9

Characteristics of Burmese Rubies