Paste is a term for leaded glass that were cut to resemble gemstones. I have read that it was more difficult to cut paste than a diamond! It took a great amount of skill and these early 19th century pieces are wonderful examples of the quality.
During their peak of popularity (1700's-1800's) paste jewels were very desirable. Due to the wide variety of cuts they could be set in jewelry that diamonds could not be. The leaded glass gives the stones a brilliant glow that you can only find in old paste. Some paste stones were foiled and there are rare pieces that were tinted to various colors.