Pectolite is a mineral of the wollastonite group with the chemical formula NaCa2Si3O8(OH). It has an RI of 1.599–1.628, with a typical spot RI reading of about 1.60 and a specific gravity of 2.81. The light blue variety of this mineral, known by the trade name “Larimar,” has only been reported from the Dominican Republic (R.E. Woodruff and E. Fritsch, “Blue pectolite from the Dominican Republic,” Winter 1989 G&G, pp. 216–225; figure 1).
We recently acquired a 3.91 ct drop-shaped cabochon, purchased in India as a Larimar imitation, that consisted of a ceramic material (figure 1). According to the vendor, the material was manufactured in China. Close microscopic examination showed white gas bubbles and blue color concentration (figure 2).
The cabochon’s spot RI reading of 1.53 and SG of 1.87 were distinctly lower than the values for Larimar. Semi-quantitative EDXRF chemical analysis gave a composition of 95% Al2O3 and 4% SiO2, with traces of SO3, K2O, CaO, Sc2O3, Cr2O3, Fe2O3, and Ga2O3. In contrast, the average composition of pectolite consists of 54.23% SiO2, 33.74% CaO, 9.32% Na2O, and 2.71% H2O.
Although the imitation material does indeed resemble Larimar, standard gemological tests such as microscopy, RI, and SG should be sufficient to distinguish it.