These converters are called vertical brooch sliders and are made to wear a brooch as a pendant. They are made specifically for brooches with a vertical bar pin on the back of the brooch. They work perfectly for any size brooch. They come with clear silicon stoppers to prevent the slider from moving on the bar pin. They come in gold or silver metal. 

If your brooch has a horizontal bar pin, I also have horizontal sliders listed in this category.