This is an article about some non-cannon D&D magic in response to Justin Alexander’s Kitchen Sinking article.
The Chrystal Marbles of Pax were a magic item I invented for one of my D&D campaigns a long time ago. Although they are fairly innocuous they became a favoured possession to my players for quite some time.
The Marbles are a pair of identical, fist sized, crystal marbles polished into smooth spheres. Whenever light enters one it exits the other as if it had entered that marble instead.
This means you can place a marble in one location and spy through it by looking through the twin marble from another location (beware, folks at the other end can look right back!). You could leave one in full daylight and have that daylight shine through the other marble while you walk through a dark location.
The marbles have a range of about 100 miles, with the light passing through being quite dim after 90 miles. Beyond 100 miles apart both marbles appear black and opaque.