Iron borate (FeBO3) is a transparent, green, uniaxial, canted antiferromagnet with a Curie temperature of 348°K. The optical absorption in the samples observed has a local minimum of 39 cm−1in the green (5250 Å). The Faraday rotation at 5250 Å is 2300° per cm. The phase retardation associated with the uniaxial birefringence is about 4 × 105per cm and the two indices of refraction are about 2.1 for the visible spectrum.