Corundum var. Ruby - Gem Tabular Ruby Cluster - Very Uncommon, Very Unique Crystal Habit
Most ruby specimens are either high-priced single/group of crystals on a marble matrix, or cheap massive habit rough from India. Not so here. This cluster of hundreds (or thousands) of intersecting crystals screams color and brightness, and has a delicate, not seen before crystal habit.
I found two pieces of this material at the 2018 Tucson Gem show. A vendor had them sitting on her table in one of the tents behind a hotel. She had no clue what they were. Under my 365nm Convoy flashlight they fluoresced brightly, the color reminding me of some of the nicest ruby specimens I had seen. But when I looked closely at the crystal habit I quickly discounted that ID; rubies simply are not commonly seen in this thin, platy habit (although the hexagonal shape made sense, and there are certainly micro-photos of such a habit). We spent the rest of the show trying to find other examples with no success. Showed them to every knowledgeable person/geologist I could find and got the same reaction - nice ruby color, but can’t be. Several folks made guesses (erythrite being a top one).
When I got home started my research on the mineral and the locality. I was able to find a few more pieces, one with a detailed locality label (but wrong ID). ID’ing was another thing; it was easy to eliminate the erythrite guess from some of the geologists using a hardness test. Erythrite is a very soft mineral. These crystals are hard and brittle. I isolated a few and crushed them against a piece of glass; they scratched the glass handily; a needle under the microscope had no effect and in fact showed that the crystals were quite hard and firm, although delicate because they are so thin. Did some research on the web and came up empty. That exhausted my ID methods. So I sent a small specimen to some folks in Europe for real testing.
The results were shocking - Thin, flat corundum crystals, var. ruby.
Full Size Images (click for larger image)
Detailed Testing and Analysis
A spectroscopic examination clearly shows the luminescence matches that of corundum. The transmission spectrum shows the typical green and blue absorption bands for Ruby. But the specimen fluoresces under a halogen lamp and you see an emission spectrum present in the transmission one. The absorption in the blue is somewhat stronger than in ruby (presence of Fe3+ or Ti3+), resulting in a more purple color. Ruby from Chumar (Nepal) has a similar purple color. It is generally accepted that any corundum with a pink, red, or purple coloration is ruby.
The emission and transmission spectrum was already very convincing but I decided to go for proof positive and send some samples off for XRF testing. The results were conclusive, showing Ai - aluminum oxide as the only element. This fits corundum.
Ruby with this thin platy structure of this size have (to my knowledge) not been marketed before (or perhaps even identified). The surface texture of the crystals is typical for corundum, and of course its ability to scratch glass; my first guess was accurate (but just lucky). The thin platy structure points to a growth in the gas phase under silica poor conditions, a volcanic origin is not impossible. In the lab corundum crystals have been grown where the crystal habit developed in thin hexagonal plate-like crystals as they grew large. Li2Mo2O7 flux was found to be a suitable flux for the growth of plate-like corundum crystals.
Most of the crystals are colored a nice reddish purple and are gemmy translucent. There are bunches of smaller crystals that are colorless and non-fluorescent (see study images below). The matrix consists of very small Ca crystals on top of what might be a massive gray rock of volcanic origin - non-fluorescent. Given their thinness the ruby crystals are quite sturdy. The flat face is the basal pinacoid, or the 0001 face - (1977 Hurlbut/Klein Manual of Mineralogy, 19th Edition). The crystals are in the Dihexagonal-Dipyramidal Class of the Hexagonal System.
Locality: I did a lot of research on this. China vendors typically don’t provide good locality information, but I was able to find a few more pieces, one from a respected USA geologist’s collection with a detailed label (but wrong mineral ID). This is the best info I have to date - will be researching further.
Large Images - a Detailed Study
(Note: some white light pics appear a different color due to the light source I used for the macro shots (halogen) on some of them.)
The pictures below were taken using a Nikon 5200 w/ macro lens, using stepped focus macro shots. Each picture is a combination of a dozen or more shots with the focus adjusted to provide a sharp macro image of wide areas. Click on any image for a scollable gallery with much larger images and an explanation of each picture.