Cerussite

Posted on September 1, 2021 by John J. Bradshaw, GIA GG

INTRODUCTION Cerussite is a mineral species, lead carbonate, with the formula PbCO3. It crystallizes in the orthorhombic system. It is an important ore of lead. The name is from the Latin, cerussa meaning white lead. The type locality is in Veneto, Italy. In historical times, cerussite was used by women to make their skin white. It was also a key ingredient in lead paint. Thankfully, both practices have long since been discontinued. Cerussite is strictly a collectors’ gemstone because of its low hardness and extreme heat sensitivity. It is, however, one of the most beautiful of all gems. Cerussite has a high refractive index, an adamantine luster and when properly faceted, is extremely bright with higher dispersion than diamond (Figure 1). FIGURE 1. Cerussite square cushion brilliant from Namibia, 14.55ct. Faceted and photo by John Bradshaw, www.rare- stone.com LOCALITIES/COLORS According to mindat.org, cerussite has been found in over 90 countries but virtually all of the largest and finest gem material is from the Tsumeb area of Namibia. Some of the more important mines include the Tsumeb Mine, the Kombat Mine and the Berg Aukas Mine. Much of the material is highly transparent, colorless, yellow (Figure 2), and light (Figure 3) to medium-dark brown (Figure 4). Occasionally, bright red inclusions of chalcotrichite make the color appear pinkish-red (Figure 5). Although rarely seen, cerussite can also produce a nice cat’s eye (Figure 6). FIGURE 2. Cerussite square cushion brilliant form Namibia, 20.18ct. Faceted and photo by John Bradshaw, www.rare-stone.com CUTTING CHARACTERISTICS Cerussite is among the most difficult stones to facet due to its low hardness, brittleness and extreme heat sensitivity. To successfully facet a cerussite takes some skill and a great deal of patience. Because of the time involved in cutting, it is not economical to cut stones less than 5 ct. Larger stones (over 20ct) are difficult as well, mostly because of the extreme heat sensitivity. A cool or cold cerussite touched with a warm hand can generate a crack within the stone. During the cutting and polishing process, this can also happen if one area of the stone generates even a small temperature change from the rest of the stone. This leads to an interesting property that very few people have heard about. In fact, the first time it happened to me, I thought I was imagining things. While cutting an approximately 30ct stone in preparation for the Tucson show, the cerussite that I was working on developed a fracture. Having no time to recut it before the show, I put it aside. At some point later that year, I went back to the stone to repair it and the crack was gone! It had essentially healed. Upon careful examination, it could still be seen but not easily. It was similar to seeing a twinning plane in a calcite or sphene. I thought that in my haste of getting ready for Tucson, that I remembered the break being worse...

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