Here’s the whole story, from Old Farmer’s Almanac:
A witch ball is a hollow sphere of plain or colored glass with glass inclusions (webs) inside the balls. Originating in 18th century England, witch balls were believed to ward off evil spirits, witch’s spells, or ill fortune. (Some historians disagree, saying the witch’s ball actually originated among cultures where witches were considered a blessing. These good witches would “enchant” the balls to enhance their potency against evil.)
When witch balls found their way across the pond to the American colonies, they were quickly adopted by superstitious folks who reasoned that evil spirits were attracted to the colored glass spheres. The spirits were drawn inside and captured in the glass web within the ball preventing their escape, and protecting the home from harm.
Legend has it that fishermen on Cape Cod placed witch balls in their nets to avoid evil spirits on the high seas. It is also claimed that Christmas ornament balls descended from the witch ball. The ball was allegedly placed on the Christmas tree to dispel a visitor’s envy at the presents beneath the tree.
They are beautiful, whether or not you take the folklore seriously.