The Burano Formation in Val Secchia in the province of Reggio Emilia is rich in black or very dark quartz. The crystals are often pitted by scars, rarely shiny, often opaque, rarely translucent, and about 3 cm long. However, they have a beautiful color and are generally euhedral and bi-terminate with simple habitus and well developed faces—for these characteristics, they are very sought after by collectors. Micro-Raman analyses showed the quartz contains abundant inclusions of anhydrite and graphite. The inclusions of anhydrite are responsible for the chromatic inhomogeneity, while the black color is linked to the presence of disordered graphite inclusions. LA-ICP-MS analyses did not show a significant presence of chromophore elements. Black quartz formed in the original evaporite deposits of gypsum, which, due to diagenesis and the increase in temperature during deep tectonic burial conditions, lost water of crystallization and subsequently transformed into anhydrite. After the formation of quartz crystals, the Burano Formation was exhumated and rehydrated, resulting in gypsification at conditions close to the surface. The black quartzes of the Burano Formation represent an appreciable gemological material for the development of local craftsmanship, even if the difficulties in reaching the deposit limit their marketing.
This work investigates the gemological properties of the datolite from the famous field of Campotrera near Reggio Emilia in Italy, for a possible commercial use in the market. This mineral occurs in widespread multi-centimeter veins, together with calcite and prehnite, within polygenic breccias in basaltic ophiolites. The most common form for this datolite is the double wedge with a prism (110) and a pinacoid (001). The gems obtained are mixed or carré cut, colorless or salmon pink, transparent, with a vitreous luster and weight between 1 to 5 carats. They have high brilliance, transparency and birefringence, glassy luster, absence of cleavage. The major chromophore is probably Fe, which occurs as inclusion of hematite and ilmenite. Raman investigations highlighted different fluid inclusions. The primary are randomly distributed or, in some cases, follow the growth zones, while the secondary form aligned tracks along the microcracks. Fluid inclusions can be biphasic and made up by liquid + gas (L + G), generally >10 mm in size, and more rarely, monophasic, composed only by liquid (L) generally <10 mm. The gems extracted from the rough sample are very valuable but their delicacy requires attention in the cutting and preparation of the jewels.
