This brooch is marked with Viennese hallmarks. This specific technique is called ‘reverse intaglio’. A reverse crystal intaglio is a cabochon cut natural crystal with an intaglio carved into the flat underside. The roses were then realistically painted, and the stone mounted on an opaque background mother of pearl. Finally, the back was sealed with gold, to preserve the painting. When viewed through the top, the three roses seem three-dimensional.
The technique originated in Belgium, c. 1860, and is attributed to an artist named Emile Marius Pradier. His method was popularised both in England and Austria. More information about reverse intaglio on this website: Lang.
Roses are probably the most popular flowers, and they are often depicted in jewellery. Red roses are often given to show love and passion, and they also symbolise respect for great courage.
The three roses on the third photo are by the impressionist Auguste Renoir (1841 – 1919).