Ive lost count the number of times ive been asked if i would like to buy Jet buttons, of course these buttons are always Glass. A common term often used is French jet buttons which of course are made of glass not jet. These glass buttons were made in France, Austria, Germany and Czechoslovakia.
Jet is a black mineral which was formed from the wood of a type of 'monkey puzzle' tree over 135 million years ago.99% of supposed jet buttons are in fact glass, true jet is a soft, black coal like substance and is light in weight, found in North Yorkshire and of course the best quality is found in Whitby. Poor examples can be found in Spain and France. Actually jet isn't actually a suitable medium for buttons as it damages easily.
In the 19th century very few true jet buttons were produced therefore most jet buttons are very rare and mostly found in museums.
Jet is more suited to the manufacture of jewellery as shown below (http://www.whitbymuseum.org.uk/collections/jett.htm)