Sugilite occurs as an essential mineral amounting to 3–8 percent by volume in aegirine syenite from Iwagi Islet, Ehime Prefecture. It is associated with albite, aegirine, pectolite and a Ca-K-Ti silicate (a still to be identified mineral). Sugilite is light brownish yellow, luster vitreous, streak white. Symmetry hexagonal. Cleavage 0001 weak. Hardness 6–6¹/₂. Specific gravity 2.80₂ (calc.), 2.74 (meas.).
Under the microscope, it is uniaxial negative with ε=1.607, ω=1.610, ω−ε=0.003, and colorless.
Estimated chemical composition is SiO₂ 71.38, TiO₂ 0.51, A1₂O₃ 2.97, Fe₂O₃ 12.76, FeO 0.19, Li₂O 3.14, Na₂O 4.37, K₂O 3.76, H₂O(+) 0.81, H₂O(−) 0.12, total 100.01%, corresponding ideally to (K, Na)((H₂O), Na)₂(Fe³⁺, Na, Ti, Fe²⁺)₂(Li, Al, Fe³⁺)₃(Si₁₂O₃₀) with Z=2.
The unit cell dimensions are a₀=10.007(2), c₀=14.000(2)Å. Space group D²_6h−P6⁄mcc. X-ray powder data resemble those for sodgianite, milarite, osumilite, merrihueite, roedderite and brannockite.
It is clear from the above-cited data that sugilite has milarite-type structure and resembles sodgianite and brannockite in containing lithium in tetrahedral site, but it is distinguished by a high content of ferric iron in 6-coordination.
The name is given in honour of the late Professor Ken-ichi Sugi (1901-1948), who first found the occurrence of this mineral with Mr. M. Kutsuna.