Sentence:  The ability of a segment of a glass sphere to magnify whatever is placed before it was known around the year 1000, when the spherical segment was called a reading stone, essentially what today we might term a frameless magnifying glass or <WSD>plain</WSD> glass paperweight. 
Meaning of the word marked with <WSD> tags:  Ordinary; lacking adornment or ornamentation; unembellished. (from 14th c.)