1570s, "reflective" (like a mirror), from Latin specularis, from speculum "a mirror" (see speculum). Meaning "assisting in vision; affording a view" is from 1650s, from Latin speculari "to spy" (see speculation).
a specular metal
spectrogram
spectrograph
spectrometer
spectroscope
spectrum
specular
speculate
speculation
speculative
speculator
speculum