Corundum

Uncommonly beautiful gems of a great variety of unusual colors, good size and sheer clarity have been sporadically mined for the past centuries from the deposits located in south-western Colombia. The fact that one of the main rivers in the area is named 'Quebrada del Rubí'('Ruby Stream'), makes it quite logical to assume that the name was related to the presence of Corundum in the surrounding area.

These deposit's materials supplies to the market have been small and sporadic therefore making them more collector's items than widely accessible commercial products.

These alluvial deposits of basaltic origin are located in a volcanic area of high mountains and steep canyons. Even after centuries of informal mining, following big rains it is possible to find, on the surface or in natural pools in the river and stream beds, material that has been washed from the surrounding formations by the flow of water and gravity. The Corundum material is usually found in association with large amounts of garnets, quartz, natural glass, hematite and olivine.

In spite of the fact that most of the crystals found have abraded surfaces, some remarkably perfect crystals have also been located, thus suggesting the possibility of an 'in situ' deposit and a potential future source of Corundum for the world markets. In the future, it would not be surprising at all that a great 'Sapphire or Ruby', resting today in any famous collection in the world and was regarded as coming from Sri Lanka, Thailand or India, is in fact a Colombian Corundum.

Gemology

Colombian corundum has gemological properties similar to corundum from other locations, in general it has the following gemological properties:

CHEMICAL COMPOSITIONAL:
2 O3  with minor traces of Cr,Ti,Fe
CRYSTAL APPEARANCE:
Great variety of natural colors observed trough the C-axis, blue to blue-green most common,colorless,red to orange,red to pink,color change material common. In general simple hexagonal prismatic crystals, tabular in shape, with flat terminations, twinning observed, smooth surfaces common, rounded terminations common also, sizes from few millimeters to 5 centimeters
REFRACTIVE INDEX:
1.762 - 1.770
SPECIFIC GRAVITY:
3.99 - 4.05 varies with the amount of solid inclusions present Inert to Short wave UV. Variable orange to red fluorescence under Long wave UV
SPECTRA:
Typical absorption spectrum for Cr,Fe,Ti elements.
ENHANCEMENT:
Characteristic corundum features hexagonal growth zoning, color zoning, twinning, several mineral inclusions commonly observed, boehmite needles, rutile crystals, elongated dark color brookite crystals, colorless transparent zircon, apatite, tourmaline, negative crystals. Blue and blue-green colors can react favorably to proper heat treatment, pink material will turn orange when exposed to high temperatures


Bibliography

AUTHORTITLEPUBLISHERYEAR
Montero M.AEstudio Mineralogico y Gemologico de los Corindones de Mercaderes-CaucaTesis de Grado Universidad Nacional,Dept. de Geociencias1.992
Themelis T.The Heat treatment of Ruby and SapphireGemlab Inc.p 156-160, 1.992
Keller P., Koivula J.I., Jara G.Sapphire from the Mercaderes-Rio mayo Area, Cauca, ColombiaGems & GemologySpring 1.985 P.20-25
Bank H. Scmetzer K. Maes J.Durchsichtiger, blau-rot changierender Korund aus KolumbienZeitschrift der Deutschen Gemmologischen GesellschaftVol.27, No.2 pp102-103, 1.978