Examples are arbitrarily aligned, optically uniaxial samples, or optically biaxial samples. This is true of either a uniaxial or biaxial material. Some materials use biaxial tensile testing. For this reason, birefringent materials with three distinct refractive indices are called "biaxial". Being triclinic, ulexite is optically biaxial. The hydraulic sheet bulge test emulates biaxial deformation conditions commonly seen in production operations. Saddle joints are said to be biaxial, allowing movement in the sagittal and frontal planes. Brookite is doubly refracting, as are all orthorhombic minerals, and it is biaxial (+). Earth is not a sphere, but an irregular shape approximating a biaxial ellipsoid. The binding energy increases as biaxial strain is applied. Some vendors of voided biaxial slabs supply prefabricated components which are quicker to install onsite. Steatite ceramics are low-cost biaxial porcelains of nominal composition (MgO)(SiO). The same memoir contained Fresnel's first attempt at the biaxial velocity law. The case of so-called biaxial crystals is substantially more complex. For the biaxial case, he generalized the surface to allow three unequal principal dimensions.