This brooch is made using a specific technique, a so-called ‘reverse intaglio’. A reverse crystal intaglio is a cabochon cut natural crystal with an intaglio carved into the flat underside. This image is painted and mounted on an opaque background (usually mother of pearl).  Finally, the back is sealed with gold, to preserve the painted areas. When viewed through the front, the image appears three-dimensional, like a flower in a miniature display. The technique originates in Belgium c. 1860 and is attributed to an artist named Emile Marius Pradier. This technique gained popularity mainly in England and Austria. More information about this technique can be found at Lang.

Link for source photo 3 and information about the medicinal properties of Gentiana species