Rubies are the red variety of corundum, bearing a very high reputation in the gem trade.The colored stone trade has benefited from global business, new mining discovery,and a new market.Tanzania has been known for producing various gems for decades,such as ruby from Winza and  Longido,  sapphire from Wuba,spinels from Mahenge,tsavorite garnet. Cabochon quality rubies have been discovered in the Longido area for more than half a century.Recently,facet-quality  materials have been produced in Longido,causing fever in the gem market in a short time.In this study,the authors will present firsthand information in the mine area and applied various testing methods to investigate the gemological features of rubies from Longido. We also make a general comparison among rubies from Longido,Mozambique,and Burma,aiming to provide certain features to differentiating the origins,disclosing more critical information, and enhancing more transparence.

Fig.1 In this study,faceted rubies from Longido Tanzania ranged
from 0.50 to 1.00 ct.,showing a nice Pigeon's Blood red color with
medium to high saturation.Photo by Yizhi Zhao.

Fig.3 In 2018,Guild Field Gemology Team documented ruby mining
activity in Longido,Tanzania.Photo copyright of Guild Institute of
Gemology.

1 MINING AND PRODUCTION

In Longido,Tanzania,Ruby was discovered in 1949 by two English  prospectors who  lived in Nairobi,Kenya, searching for minerals and gems in the African wilderness. After  weeks  of  exhausting   searching  in  this  area,one  of  the prospectors named Tom Blevins came to an outcrop of weathered green rock and noticed well-formed flat tabular and  hexagonal-shaped deep red ruby crystals laying on the rock.

The smaller ones have transparent areas and were facet grade, while larger ones still in the green rock were opaque but with their  distinctive  hexagonal shape and red color, making a striking appearance against the green host rock.Nevertheless, there are earlier accounts of ruby being  found in Longido dating back a hundred years.Mining was being conducted in the late 1950s.

Large amounts of material were initially found near the surface,but now most mining requires  extensive  tunneling. Most of the Longido ruby is found in green zoisite and dark amphibolite rock. Most are nearly  opaque and make for beautiful carving material.The ruby in green zoisite can make an amazingly beautiful and striking carving  material,which skilled carvers can make into carvings where the two opposite colors complement each other.However,a small percentage of production can be transparent facet grade.These rubies can be overly dark when cut over one carat.Still,material cut into half and three-quarter carat size faceted stones can be a beautiful Pigeon's Blood red color without needing heat treatment.

In late 2017 and early 2018,large amounts of Longido small Pigeon's  Blood red unheated facet  grade  ruby  rough entered the  Chinese market through Nairobi,Kenya.This  led Guild to travel to Longido Tanzania with Chinese and local contacts to document the mining activity and material being mined just before the 2018 September HongKong Show.

The tunnel mine,documented by Guild field gemologist Andrew Lucas, had a long history of mining and extended for hundreds of meters.After arriving in Arusha,we drove to Longido.Africa is so amazing to visit, especially Tanzania. While going to Longido,we had to stop several times for wild animals to cross.During the visit,Andrew Lucas interviewed the mine manager about the mining in 2017 and 2018.He said that in 2017 a significant pocket of tiny fine color transparent crystals was uncovered.This led to large purchases by Chinesebuyers and much more extensive mining activities.The area near the mine changed from a few huts to over 1,000 homes. Foreign investment also came into the mine to increase the mining pace and hopefully found more pockets of this quality material.

The mining work is very strenuous. This is hard rock tunnel mining. Drills and jackhammers are used to remove the ruby with some hand tools once a pocket is reached. It is difficult to move the tunnel forward through the hard rock. The larger you make the tunnel, the more time, labor, and expense is involved, so in some areas, miners have to work bent over while digging in the hard rock. There are gasses released from the earth in the tunnel mixed with the fumes from the power tools and dust in the air, making ventilation at the deeper depths difficult. While Andrew Lucas was documenting in the tunnel, two strong miners had to be evacuated out due to the fumes in their twenties. 

While in the area, Andrew Lucas also visited ruby mines being recently developed. The tunnels were still relatively shallow and were always just exploring for rubySome of these were less than a hundred meters deep. The valuable pockets found in 2017 increased interest and investment in prospecting for the next tunnel that contained the yet undiscovered pocket that would make the miners rich. 

This study characterized 91 rubies and their associated minerals in their matrix by using various techniques. Slices were cut from the wall rocks and polished to analyze the associated minerals. Most of the rubies are highly saturated with a bright to the deep tone of red color, weighing from 0. 50 ct to 1. 00 ct in rough and 0. 50 to 1. 00 ct faceted stones, as shown in Figure 1. Details of testing are described in Table 1. 

At Guild Gem Laboratories, refractive  indices  and birefringence were measured on a refractometer with a near- sodium equivalent light source. Specific gravity was calculated by  the  hydrostatic  weighing  method. Pleochroism  was observed using a handhold dichroscope. Fluorescence reaction was observed in a dark room under standard long-wave (365nm)and  shortwave(254nm)UV  radiation. We  used  a standard gemological microscope, equipped with Leica optics with magnification up to 80×, to observe the internal features. 

The chemical compositions of minerals were analyzed using Laser Ablation Inductively Coupled Plasma Mass Spectrometry(LA-ICP-MS)at the Sample Solution Laboratory in Wuhan, China. A193 nm ArF Excimer laser ablation system (GeoLasPro), coupled to an Agilent 7700 ICP-MS, was used for trace element analyses. The spot diameters were set to be 44 μm with laser energy at 80mJ and laser frequency at 5 Hz. Helium served as a carrier gas. Nitrogen was added into the central gas flow(Ar+He)of the Ar plasma to improve the  detection  limit  and  precision. The  USGS  standards (BCR-2G, BHVO-2G, BIR-1G)were  used  as  an  external standard, and NIST610 was analyzed every six analyses for time-drift  correction. Off-line  selection  and  integration  of background and analytical signals, and time-drift correction,  and quantitative calibration for trace element analyses were performed by ICPMSDataCal12-37

Standard Gemological Analysis. 

Several ruby mines have been found in Tanzania, and their gemological features were studied in previously  published work, such as Winza⁴]and Morogoro . In this study, the gemological characteristics of ruby from Longido were typical for corundum in general and similar to those of African rubies previously observed. Details on the gemological features of Rubies in this study can be found in Table 1 below. 

Wall Rock and Crystal Habit.

 In  this  study, the rough rubies are mainly in the subhedral or anhedral shape from the perspective of the crystal habit. Euhedral hexagonal crystals were also observed within the feldspar matrix. Among the roughs, fragments were commonly seen with subhedral shapes. They were made up of a collection of various crystal faces, mainly basal plane(0001), hexagonal prism(1010), and rhombohedra(10i0). Simultaneously, some well-formed crystals also existed with crystal faces mentioned above, mainly with a platy shape due to the excessive development of rhombohedral crystal faces. For the wall rocks, ruby crystals embedded in the green to dark green zoisite, white feldspar, and dark amphibole(Figure 5-6). Details will be discussed in the associated mineral part. Generally, most of the rubies
were in high red color saturation with an attractive appearance of a high degree of transparency, strong luster, and brilliance exhibited in cut stones.

Fig.5 Rubies within the matrix,associated with green zoisite,deep green amphibole,white feldspar,and green mica.

Photo by Yizhi Zhao.

Fig.6 The wall rock specimen was cut into two parts by author Peter
Gao,showing the fresh face and distinct color distribution.Photo by
Yizhi Zhao;Vertical illumination.

Associated Minerals and Inclusions. 

Pargasite Amphibole. Pargasite belongs to the amphibole group with an ideal chemical composition of NaCa₂(Mg, Fe²+)4(Al, Fe³+)(Si₆Al₂) O2₂(OH)₂. The presence of pargasite is a helpful indicator that Longido ruby formed in amphibole-hosted deposits, similar to those found in Mozambique(Figure 7a). 

Zoisite. 

Zoisite mainly formed together with pargasite as coarse grains in the matrix(Figure 7b). The previous production material was  cabochon quality, and the  part containing ruby and zoisite is usually fashioned as carving to show the combination of red and green color. So, it is not surprising to find zoisite with ruby. 

Labradorite Feldspar. 

Labradorite is one member of the plagioclase series in the feldspar group, and ruby crystals were found adjacent to white color. Labradorite, together with some greenish-blue color mineral. Labradorite exhibits distinct interference color and perfect cleavages when viewing under reflected light(Figure 7c)

Paragonite Mica.

 Paragonite is an uncommon mineral. It could be found in large amounts, such as in low to medium- grade  metamorphic  schists  and  phyllite, in  muscovite- biotite  gneisses, quartz  veins, fine-grained  sediments, and glaucophane-bearing rocks⁶. In this study, paragonite minerals were found within the feldspar matrix, exhibiting bluish-green color and silky luster (Figure 7d). It is essential to point out that all these green paragonites are only found in the reaction zoning between ruby and feldspar. 

Fig.7 Associated minerals
(a:Pargasite of deep green body colorexhibits sub-prismatic crystal habit.

The dark tone of the body color maybe attributed to the high content of iron.

b:Granular zoisite minerals usually occur with deep greenpargasite.
c:Labradorite-associated mineral in the matrix.

d:Paragonite with bright bluish green color always occurs by the rim of rubies,as well as feldspar.
Photomicrographs by Yizhi Zhao;Vertical illumination.)

Inclusions. 

Rutile. As a typical  guest, rutile  inclusions have been found in rubies from many localities, such as Burma, Mozambique, Sri Lanka, and rubies from Longido in this study. Rutile is an oxide mineral composed of Ti and O. Such mineral inclusions usually scatter in Longido ruby in the form of platy and short needles, as shown in Figure 8a.

Diaspore.

 Diaspore is a hydro-aluminum oxide that is commonly seen in corundum. It is usually distributed on a submicroscopic scale and not easy to be observed by the naked eye or even under high magnification under microscopic. However, in this study, we have found a transparent colorless subhedral crystal near the girdle of one faceted ruby sample (Figure 8b), confirmed as diaspore by the Raman spectrum. 
Growth structures. Sharp and straight growth lines in a
hexagonal pattern were usually observed in most samples in this study(Figure 8c). Within the growth  structures, clouds were composed of numerous minute particles as well as triangular platy rutile. 

Fluid inclusions. 

Fluid inclusions were less commonly encountered in the samples studied, only seen in several samples(Figure 8d). These fluids were mainly composed of CO₂and in the healed fractures. 

b:Transparent colorless crystal near the griddle of one ruby sample was confirmed as diaspore by Raman. Darkfield illumination.

c:Straight and hexagonal  growth structures were commonly observed in these Tanzania rubies,  

usually accompanied by minute cloudy inclusions and platyrutiles. Darkfield illumination.

d:Fluid inclusion along the healed fissures  was also observed in those Longido Rubies.Darkfield illumination.

Chemical Composition.

Owing to the similarities to Al, elements such as Fe, Cr, Ti, Ga, et al. can enter the crystalline structure of the corundum and substitute the position of Al and be part of the corundum structure. The variety and content of these substituting cations have a solid relationship with the ambient environment of corundum. Since ruby can form in various geology settings and occurrences, such as marble in Burma, amphibole in Mozambique, and basalt in Thailand/ Cambodia, trace elements can be beneficial in differentiating ruby from different localities. Several elements could be helpful, including V, Cr, Ga, Ti, and Fe. The similarities of these elements with Al allow them to enter the crystalline structure of the corundum. However, the ambient environment where ruby is formed may contain these elements at different levels. Hence the trace element contents and their relative ratios can be indicators for origin determination. Take Burmese, Mozambique, and Longido ruby for example, Fe and Cr contents can show a clear difference among these three origins (Figure 9). 

Fig.9 Plotting of Fe and Cr shows the difference of rubies from Mozambique,Burma,and Longido Tanzania in this study.

4 Conclusion

Rubies from Longido, Tanzania, are found with wall rock composed mainly of feldspar, mica, zoisite, and amphibole The high transparency and clarity enable the stones as faceted-grade instead of carving or cabochon. The presence of boehmite may act as solid proof of the absence of heating. High chromium and low iron make the color more saturated in red and a valuable parameter to differentiate them from Burmese and Mozambique rubies. 

The relatively large production allows the cutters to perform more exciting and new cutting styles and shapes to give designers more options. The new product may provide more good quality material for jewelry, especially for rubies of small size. However, the sustainability and stability of supply are still unknown and remain to be seen. As always, we believe that knowledge could help to build trust, especially in the gem industry. Guild Research  team will  focus on  ruby  from any localities and bring the latest knowledge in the future. 

Fig. 10A mesmerizing pendant is featuring a ruby from Tanzania. Photo by Kaiyin Deng, courtesy of PADOBO.