I could not resist. This Tweet displays the crown of the Holy Roman Empire. When I show slides of Charles VI and Maria Theresa in class, this is one of the crowns sitting on the table beside them.
Why were the Habsburgs so rarely depicted as wearing this crown? Among other things, as a practical matter its too heavy to wear very long. Also, although the imperial crown is the most important in Europe, the Habsburgs also had the right to wear other significant crowns. They had a separate crowns as kings/queens of Hungary (the crown of St. Stephen) and as King/Elector of Bohemia.
You can see this crown on display in the Treasury (Schatzkammer) in the Hofburg Palace in Vienna. Another reason to visit!