Nature sometimes plays a funny game with our senses. Some mineral formations may remind us of completely ordinary things. Just think of the formations in stalactite caves, which are often given separate names (e.g., Great Church, Santa Claus, Dragon’s Head, etc.). In Harghita, in the surroundings of Herculian, geysers were active in the past. From their water, white opal and brown siderite are deposited alternately. The latter was partially converted to goethite by oxidation. The resulting layered structure reminded the geologist János Bányai (1886-1971) of the “Dobos” cake (a popular cake in Hungary), where the opal layers symbolize the “sponge sheets” and the siderite layers symbolize the “chocolate cream” and “burnt sugar”. The name “Dobos cake” opal, created in 1933, has survived to this day.
In the picture: “Dobos cake” opal, Herculian (Romania). Size: 7 cm. Photo by Géza Kulcsár.