b'GarnetFeatureNominis unde sui causam traxisse videtur,Sed Graeca lingua lapis idem dicitur anthrax.Huius nec tenebrae possunt extinguere lucem;Quin flammas vibrans, oculis micet aspicientium.On the carbuncle.The carbuncle outdoes all the flashing gemsFor it emits rays in all directions as if a glowing emberFrom which effect it seems to have acquired the name.In Greek it is called the anthrax.Darkness cannot extinguish the light of this stoneIndeed it twinkles, to gleam in the eyes of observers.In 16th century Europe garnets were admired minerals, particularly in Germany. Georgius Agrocola (1494-1555), the German Georg Bauer, was born in Saxony during a time of the European revival of classical learning. His mineralogy book De Natura Fossilium established him as the Father of Mineralogy (DNF, 1546; translated by Bardy and Bandy, 2004). Book 6 of DNF devotes several pages to garnets, their properties and localities. Brilliant red carbunculi were particularly esteemed.In England gemmy garnets were known as carbuncles during and after the rein of Elizabeth I (1558-1603). It was believed that carbuncles illuminated themselves when in darkness. So in Shakespeares Hamlet (1603) and Miltons Paradise lost (1667) the eyes of unsavoury scary characters glow like carbuncles (Satan/Milton, Pyrrhus/Shakespeare). In this period affordable garnet was a favourite gemstone especially for rings and inlays. These garnets came from Bohemia where they still reside in Figure 4.Faceted backlit pyrope from Kenya, National Museum Washingtonextensive alluvials shed from a peridotite host (Voynick, 2021).(top), CC BY SA 2.5 / Thomas Ruedas/ https://commons.wikimedia.org/wiki/File:Pyrope.jpg. Orange striated gemmy spessartine on smoky quartz crystals,Garnets reputation as a popular semi-precious gem continues ~60 mm lateral view, Wushan Mine, Fujian province, China (middle) Robtoday. Reddish almandine and pyrope are still referred to as the Lavinsky, iRocks.comCC-BY-SA-3.0. Sharp edged rhodolite eluvial floaters fromcarbuncle stones in some quarters (Schumann, 2006).Rift Valley Province Kenya (bottom) Rob Lavinsky, iRocks.comCC-BY-SA-3.0. These pyrope-almandine (2:1) sharp edged rough specimens are rosy red when frontlit (as here) and are cherry red when backlit, each ~20mm lateral dimensionGarnetdensity and magnetic susceptibilityand ~7ct mass (bottom).Garnets are much studied by solid earth geophysicists and petrologists whose interests lie in the deep crust and upper Pliny continued to elaborate on particularly attractive garnets,mantle where garnets are a significant constituent. Electrical especially what seems to be the violet tinged almandine,and thermal conductivity, P and S wave velocities, mechanical apparently from Myannmar: properties have received considerable laboratory attention, in masculis quoque observant liquidiores aut flammaeunder high temperature and pressure conditions. However, nigrioris et quosdam ex alto lucidos ac magis ceteris in solefor near surface garnets the two basic properties of interest flagrantes, optimos vero amethystizontas, hoc est quorumto exploration geoscientists should, in my view, be density (D) extremus igniculus in amethysti violam exeat. and magnetic susceptibility (k). The relationship between these two basic properties has received little attention. Garnets are also, among the male stones, one makes special note of thosevariably dense, and some display a moderate paramagnetic that: are quite transparent, glow with more dusky character,susceptibility. Table 1 gives nominal values of these quantities. and some that shine from inside, become quite fiery in the sun,Figure 1 shows densities and the variability which results and for sure the best are those with an amethyst tinge being thefrom any garnet having at least part of another garnet in its type where a violet glitter develops at the edge of a (red) stone. structure owing to isomorphous mixing, and also owing to the Modern connoisseurs of gemmy garnets could only agree withinclusion of other silicates and oxides. For example, a nominally Plinys preferences. almandine garnet, from thermal metamorphism of a pelite, could actually be Alm 78 Py 15 Sp 4 Gro 3by solid solution mixing Garnet gems were admired in the Middle Ages. Marbod (1035 (Deer et al 1992) and include rutile needles (TiO 2 , 4.25 g/cc) and 1123), Bishop of Rennes in Brittany, celebrated (and somewhatzircon grains (ZrSiO 4 , 4.68 g/cc). So, the resulting density will exaggerated) the merits of garnets in the 23rd Latin poem of hisnot be almandines Table 1 value, 4.32 g/cc. Hence the variability famous lapidary (compiled by Beckmann, 1799): shown in Figure 1 for natural garnets. The Table 1 nominal values have been used in data cross-plotting (following) but the De Carbunculo. heterogeneity of any garnet needs to be kept in mind in any Ardentes gemmas superat carbunculus omnes; assessment. For similar reasons magnetic susceptibilities will Nam velut ignitus radios iacit undique carbo, vary too.DECEMBER 2021 PREVIEW 66'