Laurentthomasite, ideally Mg₂K(Be₂Al)Si₁₂ O₃₀, is a new milarite-group member found within quartz-syenite pegmatites from the Ihorombe region, Fianarantsoa Province, Madagascar. It occurs as euhedral {0001} hexagonal crystals, maximum 15 mm large and 5 mm thick. The crystals show a very strong dichroism with cobalt blue and green-yellow colours when observed along [0001] and [1000], respectively. The mineral is transparent, uniaxial (+) and its lustre is vitreous. The hardness is about 6 (Mohs scale), showing a poor {0001} cleavage, irregular to conchoidal fracture, and a measured density of 2.67(8) g cm⁻³. Laurentthomasite is hexagonal, space group P6/mcc (no. 192), with a=9.95343(6) Å, c=14.15583(8) Å, V=1214.54(1) ų and Z=2. The strongest nine lines in the X-ray powder diffraction pattern [d in Å – (I) – hkl] are 3.171 – (10) – 211, 4.064 – (8) – 112, 2.732 – (8) – 204, 4.965 – (6) – 110, 2.732 – (4) – 204, 3.533 – (3) – 004, 7.055 – (2) – 002, 4.302 – (2) – 200 and 3.675 – (2) – 202. Chemical analyses by electron microprobe and several spectroscopies (inductively coupled plasma, ICP; optical emission, OES; mass, MS; and Mössbauer) give the following empirical formula based on 30 anions per formula unit: (Mg_0.86 Sc_0.54 ${\mathrm{Fe}}_{\mathrm{0.35}}^{\mathrm{2}+}$ Mn${}_{\mathrm{0.26}}{)}_{\sum =\mathrm{2.01}}$ (K_0.89 Na_0.05 Y_0.02 Ca_0.01 Ba${}_{\mathrm{0.01}}{)}_{\sum =\mathrm{0.98}}$ [(Be_2.35 Al_0.50 Mg_0.11 Fe${}_{\mathrm{0.03}}^{\mathrm{3}+}{)}_{\sum =\mathrm{2.99}}$ (Si_11.90 Al_0.10)O₃₀]; the simplified formula is (Mg, Sc)₂(K, Na)[(Be, Al, Mg)₃(Si, Al)₁₂ O₃₀]. The crystal structure was refined to an R index of 1.89 % based on 430 reflections with Io > 2σ(I) collected on a four-circle diffractometer with CuKα radiation. By comparison with the general formula of the milarite group, A₂B₂C[T(2)₃T(1)₁₂ O₃₀](H₂O) x ($\mathrm{0}<x<n$ , with n<2 pfu, per formula unit), the laurentthomasite structure consists of a beryllo-alumino-silicate framework in which the T(1) site is occupied by Si and minor Al and forms [Si₁₂ O₃₀] cages linked by the T(2) site mainly occupied by (Be + Al). The A and C sites occur in the interstices of the framework while the B site is vacant. The origin of the strong dichroism is related to a charge transfer process between Fe²⁺ and Fe³⁺ in octahedral A sites and tetrahedral T(2) sites, respectively.

In this study, a rare 3.49-carat yellow diamond was analyzed to reconstruct the geological processes that led to its distinctive form. The diamond exhibits growth and dissolution features, indicating a complex history. To preserve the sample’s integrity, non-destructive analytical techniques—including VIS, UV–Vis–NIR, and IR spectroscopy—were employed. The yellow coloration of the diamond is attributed to the presence of N3 and N2 defects. Additionally, other defects such as N₃VH⁰ centers and platelets were detected; however, the latter do not contribute to the coloration. The observations of the etch pits and surface microreliefs suggest that the diamond underwent size reduction due to dissolution events, which also altered its crystal habit over time. The diamond’s initial mixed-habit morphology evolved into a more complex one through a series of growth and dissolution processes that began during mantle storage. Furthermore, the presence of brown surface stains indicates radiation damage, likely acquired during its residence in alluvial deposits at the Earth’s surface.