Final answer:
Opal is a form of amorphous silica, whereas quartz is the crystalline form of silicon dioxide. Opal lacks a crystalline structure and instead has a disordered arrangement of particles, giving it its unique play-of-color.
Step-by-step explanation:
Opal differs from crystalline quartz because it is a form of amorphous silica whereas quartz is the usual crystalline form of silicon dioxide.
Quartz is a hard, brittle, clear, colorless solid, while opal is a naturally occurring form of amorphous silica.
Unlike quartz, opal lacks a crystalline structure and instead has a disordered arrangement of particles.
This disordered structure gives opal its unique play-of-color, where it can exhibit a variety of vibrant colors.