Martin, it may be an ok glass one.
Do you have, or have access to, the Peter Von Brackel book - Paperweights, Historicism, Art Nouveau, Art Deco, 1842 to Present? If so, check plate 57, page 183, item 404, which seems to be the same shape facet cutting as yours and also has a fairly convincing butterfly. It's attributed as "Bohemia / Czechoslovakia, c 1900-1930". That one is set over coloured frit with a few flower petals.
Two examples with different facet cutting, are seen in Plates 56 & 59. These both have butterflies with antennae very much like yours. The first (item 401) is said to be "Probably Southern Bohemia or Hungary, c 1910-1930". The second (item 416) is, "Probably Silesia or Thuringia, c 1890-1925".