Andradite, variety demantoid, is a rare gem mineral. We describe gem-quality garnet crystals from serpentinized harzburgites from Dobšiná, Slovakia which were faceted. Both the andradite samples were transparent, with a vitreous luster and a vivid green color. They were isotropic with refractive indices >1.81. The measured density ranged from 3.82 to 3.84 g·cm⁻³. Andradite var. demantoid appeared red under Chelsea filter observation. Both samples contained fibrous crystalline inclusions with the typical “horsetail” arrangement. The studied garnet had a strong Fe³⁺ dominant andradite composition with 1.72–1.85 apfu Fe³⁺, Cr³⁺ up to 0.15 apfu, Al³⁺ 0.03 to 0.04 apfu, V³⁺ up to 0.006 apfu substituted for Fe³⁺, Mn²⁺ up to 0.002 apfu, and Mg up to 0.04 apfu substituted for Ca. Raman spectrum of garnet showed three spectral regions containing relatively strong bands: I—352–371 cm⁻¹, II—816–874 cm⁻¹, and III—493–516 cm⁻¹. The optical absorption spectrum as characterized by an intense band at 438 nm and two broad bands at 587 and 623 nm and last one at 861 nm, which were assigned to Fe³⁺ and Cr³⁺. Transmission was observed in the ultraviolet spectral region (<390 nm), near the infrared region (700–800 nm), and around 530 nm in the green region of visible light, resulting in the garnet’s green color.

We studied 12 crystal fragments of natural spinel from Mogok, Myanmar and Lục Yên, Vietnam. All samples were crystal fragments of various shapes and sizes and several of them had gemological quality. Studied samples are enriched in Cr, V, Fe²⁺, Fe³⁺, Zn, which are responsible for its resulting color. They could be divided into groups of V-Cr spinels with Cr 0.001–0.006 apfu, V 0.001–0.004 apfu, and Fe spinels containing increased Fe²⁺ (0.001–0.017 apfu) and Fe³⁺ (0.004–0.012 apfu). Some samples show luminescence bands at 677, 685, 697, 710, and 718 nm assigned to Cr³⁺. The optical absorption spectra of spinels were divided into two groups of V-Cr and Fe spinels based on the dominant element acting on optical spectra. The optical spectrum of V-Cr spinels can be divided into two zones (1) 420–550 nm (V³⁺ and Cr³⁺ absorption); (2) 640–1000 nm (Fe²⁺-Fe³⁺ charge transfer). The optical absorption spectra of Fe spinels can also be divided into two zones (1) 410–650 nm (Fe²⁺-Fe³⁺ charge transfer) and (2) 770–1000 nm (Fe²⁺). This variation in chromophores results in the differences in color: V-Cr spinels are pink to red, Fe spinels are in shades of blue as well as yellow and pink.

In talc-magnesite veins in serpentinite near Muránska Dlhá Lúka (MDL), Slovakia, yellow euhedral to subhedral crystals apatite of a gem quality occur. It has a composition of hydroxylapatite with F⁻ varying between 0.29 and 0.34 apfu, Cl⁻ in range of 0.02–0.05 apfu and calculated OH⁻ content between 0.62–0.68 apfu. Moreover, [CO₃]²⁻ molecules were identified by FTIR and Raman spectroscopy. MDL apatite contains only up to 0.003 apfu As⁵⁺ and Si⁴⁺ substituting for P⁵⁺; Ca is substituted by small amount of Na, Fe²⁺, Mn²⁺ (all up to 0.006 apfu), and Rare Earth Elements (REE—in total up to 0.017 apfu). Compared to trace-element composition of similar apatites from Durango, Mexico, the REE content in MDL apatite is around ten times lower with Nd > Ce >> La, its chondrite-normalized REE pattern has almost a horizontal slope and larger negative Eu anomaly. The MDL apatite is richer in Mn, Pb and Li, but poorer in As, V, Th and U. The concentrations of Sr and Y are similar. In the optical absorption spectra, the most prominent bands are at 585–590 nm (Nd³⁺) and between 600 and 800 nm (Mn²⁺, Ce³⁺-SiO³⁻ photochromic center and Nd³⁺). The photoluminescence spectrum of MDL apatite shows bands between 550 and 620 nm (Dy³⁺, Sm³⁺, Pr³⁺ and also Mn²⁺) which likely enhance its yellow color. MDL hydroxylapatite likely formed from fluids derived from granitic rocks as evidenced by the chondrite-normalized REE patterns, Li, Mn and Y concentrations. The Sr content reflects the host-rock serpentinite composition. Fluids for its crystallization were likely derived from Muráň complex orthogneisses by the Alpine deformation and recrystallization in greenschist to lower amphibolite facies.