Kugel Christmas Ornaments
Although any round, glass Christmas ornament is sometimes mistakenly called a "Kugel", the word more accurately refers to a particular type of 19th century glass ornament.
The earliest Kugels were brought to America from Germany in the 1860's. These were thick, often very large, glass ornaments with a dull sheen and large, flat and frequently decorative caps. They are clear or colored glass which is silvered on the inside. Round ones are the most common, but there are also elongated kugels and kugels in the shape of a bunch of grapes.
There is no "stem" on the top of a Kugel as there is on modern ornaments, and the insides are silvered with lead or zinc. Except for the smallest kugels, many of the earliest Kugels were too heavy to hang from a tree!
Innovations in glassmaking made it possible to produce these large ornaments from thin glass with the mirror-like shine of present-day Christmas ornaments. Production of the old style Kugels ended by the beginning of the 20th century.
Be aware that modern reproductions and replicas are plentiful and sometimes listed as antiques. There are some great tips for spotting the fakes (and a few photos of modern reproductions) in this eBay community thread.