Apatite [$\text{Ca}_5(\text{PO}_4)_3(\text{F,Cl,OH})$] is uniaxial negative — its ordinary refractive index ($\omega$) is greater than its extraordinary index ($\epsilon$): $\omega > \epsilon$, so $\omega - \epsilon < 0$. In contrast, Quartz and Zircon are uniaxial positive ($\epsilon > \omega$). Uniaxial minerals have one optic axis (tetragonal, trigonal, hexagonal systems). The sign is determined using the Biot-Fresnel rule or interference figure with a gypsum plate.