b'GarnetFeatureresistivity, magnetics, and seismic refraction were employedGood quality andradite and its rare green variety demantoid successfully after heavy mineral alluvial and eluvial sampling(Cr chomophore) have dispersion coefficients (0.057) programs (Hausel et al, 1979, see also Jenke 1983, and Smith etexceeding that of diamond (0.044). Dispersion gives a stone al 1990). fire or colour flashes; the coefficient of dispersion being the difference between the refractive indices of red and violet light. Strong colouring in andradite and demantoid tends Gem garnets to diminish the effect, but it can be seen in weaker tints. Apart from diamonds, gemstone mining is a multi-billion-dollarThe andradites are lively as they have the highest R.I. of industry worldwide. Ruby, opal, jade, lapis lazuli, sapphire, andany garnet and hence better reflectivity and a strong sub-emerald are most valuable and hence have been the mainadamantine lustre compared to the duller vitreous lustre of exploration targets, but gems of lower value, such as garnet, arecommon almandine. Demantoid, found in serpentinites, is the still sought and sold. most precious garneta good specimen would rival emerald in value.Garnets have a long history as semi-precious gems, used in adornments, and plentiful in many colours. NowadaysIn the last resort the value of a garnet will depend on the they are still fancied as one of the cheaper gems, althoughbuyers appreciation of a stones clarity, colour, and character. No many consider them old fashioned. Gem garnet was neverother mineral has garnets range of colour; only a few surpass more popular than in the Victorian era when countlessit in character. The point of all this is to make geophysicists rings, bracelets, necklaces, and brooches were fashionedaware that garnet could be a nice little supplementary earner from Bohemian reddish-brown pyrope-almandine. Manyif, in any spare personal time they have in their travels, they survive to this day in antique and bric-a-brac shops andcare to browse around garnet areas, to meet and deal with as household hand-me-downs (Figure 7). Garnet is stilllocal fossickers, and attend local gem shows. There are bargains collected for jewellery purposes in several countries, oftento be had and good material can be on sold especially if, as water worn pebbles in alluvials, or extracted from aas geoscientists, they can add to the information about a host rock matrix, to be cut into various shapes, then to bestone with a simple density measurement and observations marketed and sold (Figure 8). While small gemmy garnetssuch as geological provenance. A big garnet with good usually are not of high value, they are more valuable thanrhombic dodecahedral geometry could weigh ~ 1 kg and be the garnet crystal trophy specimens sought by collectors,worth ~ $100, so too would be the value of a couple of grams of museums, and researchers. For such single crystals of thegood gemmy garnet rough.common almandine type, in fair to good condition, pricesA strong light source, a 10x loupe, a scriber, and a good can range from $10 for a small specimen (~10mm diameter)magnet are indispensable tools to employ in inspecting any to over $100 for a large rhombic or trapezoidal faced crystalmineral or rock, anywhere. However, when looking at gems (~100mm diameter) i.e., roughly ~ $0.1 per gram. Gemmythese may not be enough to distinguish paste i.e. glass rough of any type can be worth around $50 per gram ($10imitation of minerals and gems passed off as the genuine per carat) or more. Cutting and polishing rough into anarticle to unsuspecting buyers. Pliny commented on the skill attractive stone can increase the value by an order or ordersand cunning of glassworkers in making counterfeit carbunculi of magnitude. Some stones can be extremely valuablein first century Rome. Some glass garnets were even made (Figure 9). with internal flaws to simulate real mineral inclusions. Pliny pointed out that this could be detected because glass is Figure 6.Diamond indicator minerals from drainage sampling in kimberlite exploration Colorado, USA: purple pyrope, red almandine and orange spessartine garnet, emerald green chromian diopside, coated metallic picroilmenite, and metallic octahedral chromite. The finding of mineral suitesFigure 7.Red-brown almandine garnets with fair colour and moderate lustre such as these lead to follow up geophysics in diamond exploration programmesare here set in a bracelet and pendant cross. Inexpensive jewellery of this type https://gem-garnet.blogspot.com/. was and is still popular. It is readily available and sold worldwide (scale - cm).69 PREVIEW DECEMBER 2021'