Question

Is the erbium transparent, translucent or opaque?

Answer 1

Objects that allow all of the light to pass through them is said to be transparent. One can clearly see through the transparent objects.  Objects that allow only some light to pass through them is said to be translucent. One can partially see through the translucent objects whereas  objects that do not allow any light to pass through them is said to be opaque. One cannot see through the opaque objects at all.

Erbium is a chemical element with atomic symbol, $Er$ which belongs to the period 6 with atomic number 68. It is a lanthanide, which is silvery-white solid metal in nature. The metals are opaque to visible lights.

Hence, erbium is opaque.