Calciborite was found as a veinlet or a mass in crystalline limestone associated with gehlenite-spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Calciborite occurs as milky white aggregates up to 1 mm in diameter with shimazakiite, fluorite, bornite and calcite. An electron microprobe analysis of calciborite gave an empirical formula (Ca_0.999Mn_0.001Co_0.001)_Σ1.001B_1.999O₄ based on O = 4. The unit cell parameters are a = 8.373(2), b = 13.811(8), c = 5.012(4) Å. The mineral is optically biaxial (-), α = 1.594(2), β = 1.654(2), γ = 1.672(2) and 2V_calc. = 56°. The Vickers microhardness is 177 kg mm^-2 (50 g load), and the Mohs hardness number is 3.5. The measured and calculated densities are 2.88(2) and 2.881 g cm^-3, respectively. The calciborite from the Fuka mine was probably formed by a reaction of boron-bearing fluids with limestone at a temperature between 250 and 300 °C.