The site preference for each cation and site in beryl based on bond-length calculations was determined and compared with analytical data. Tetrahedral SiO₄ six-membered rings normally have no substitutions which results from very compact Si⁴⁺–O bonds in tetrahedra. Any substitution except Be would require significant tetrahedral ring distortion. The Be tetrahedron should also be negligibly substituted based on the bond-valence calculation; the tetrahedral Li–O bond length is almost 20% larger than Be²⁺–O. Similar or smaller bond lengths were calculated for Cr³⁺, V³⁺, Fe³⁺, Fe²⁺, Mn³⁺, Mg²⁺, and Al³⁺, which can substitute for Be but also can occupy a neighboring tetrahedrally coordinated site which is completely vacant in the full Be occupancy. The octahedral site is also very compressed due to dominant Al with short bond lengths; any substitution results in octahedron expansion. There are two channel sites in beryl: the smaller 2b site can be occupied by Na⁺, Ca²⁺, Li⁺, and REE³⁺ (Rare Earth Elements); Fe²⁺ and Fe³⁺ are too small; K⁺, Cs⁺, Rb⁺, and Ba²⁺ are too large. The channel 2a-site average bond length is 3.38 Å which allows the presence of simple molecules such as H₂O, CO₂, or NH₄ and the large-sized cations-preferring Cs⁺.

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.

A complex crystal-chemical investigation based on spectroscopic methods, Electron MicroProbe Analysis (EMPA), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was made on sphalerite samples from the Terézia and Rozália veins in the Banská Štiavnica ore district. The yellow (sample A) and olive-green sphalerite (C) are in association only with quartz; orange sphalerite (B) is associated with quartz and chalcopyrite; and black sphalerite (D) is associated with galena, chalcopyrite, quartz, and baryte. EMPA revealed that Cd and Fe are substituting for Zn with variable proportions; the Cd/Fe ratio decreases from 2.82–2.85 in the A sample to 0.42 in the D sample. LA-ICP-MS showed that, except Cd and Fe, only Mn has content above 20; Co and Cu vary between 2 and 17 ppm. The optical absorption spectra exhibit absorption between 644 and 740 nm with three smaller humps at 669–671, 698–702, and 732–743 nm, and weaker absorption bands at 858–894 nm in the NIR region, which can be all assigned to crystal-field transitions of Fe²⁺. The absorption edge starts at about 600 nm to the UV region. Minimal absorption is in the yellow-red part of the visible spectrum giving rise to yellowish-orange and orange-red colors. Absorption in the red region for olive-green sphalerite is more pronounced, explaining the shift to greenish hues. In black sphalerite, the absorption pattern is similar to the olive-green sphalerite, but the bands in the 644 to 740 nm region are less defined. The black color could be caused by slightly higher concentrations of Fe, the smaller size of individual crystals in the aggregate reducing macroscopic transparency, and/or the presence of submicroscopic inclusions.

Topaz is frequently subjected to heat treatment and irradiation to enhance colour, particularly to produce the market’s most preferred salmon pink and sky blue varieties. However, an insufficient description of these processes can lead to fraudulent practices. This experimental and forensic mineralogical and gemmological study investigates eighteen heat-treated topaz samples from Ouro Preto (OP) and Caraí (CA), Brazil, using electron microanalysis, LA-ICP-MS, Raman, and optical absorption spectroscopy before and after heat treatment at various temperatures. The most significant optical changes were observed at 300 °C when the CA sample lost its colour from sky blue to colourless, while OP samples retained their imperial orange colour up to 500 °C before transitioning to pink at 700 °C. Chemically, the CA samples are rich in F (> 1.8 apfu) with low trace element concentration (Fe ≤ 125 ppm, Ge ≤ 153 ppm), falling to the pegmatite and greisen field of topaz origin. The OP samples contain less F (1.4–1.5 apfu) but higher trace element contents (Cr up to 204 ppm, Ti up to 115 ppm, Fe, Mn, Ge < 64 ppm), consistent with a hydrothermal origin. Raman spectra show no significant inter-sample variation, but their luminescence spectra feature strong differences: Mn acts as the luminophore in CA samples, while Cr³⁺ centers dominate in OP samples. The optical absorption spectra reveal distinct thermal responses. The OP samples heated to temperatures ≥ 500 °C developed new absorption bands at 530–532 nm, consequently resulting in a visible pink colour. On the other hand, the CA spectra exhibit strong absorption in the NIR region; the unheated sample has a broad absorption band at 634 nm, responsible for the sky-blue colour of topaz. Heating ≥ 300 °C eliminates the transmission window in the blue to cyan regions, removing blue colouration. These thermal-optical signatures serve as indicators of heat treatment in topaz declared from these two localities. Moreover, the combination of spectroscopic methods, which we successfully applied in recognizing heat treatment on the studied samples, provides a systematic approach for identifying treatment in topaz and potentially other gemstones.