The sign of elongation is positive or negative depending on the orientation of its highest refractive index to the long axis of the partical. If the highest refractive index is approximately parallel to the long axis the the sign of elongation is positive. For biaxial crystals it is posible for the beta refractive index to be close to the long axis, as in wollastonite, and the particle may show both positive and negative signs of elongation depending on its rotational orientation.