Good job on your English!! 925 is Sterling Silver. (Have you had to polish the silver? Does it tarnish?)
The pearl looks a lot like a "blister pearl" to me, as the backing appears to be a cut part of the shell that the pearl grew on.
See the definition:
Blister Pearl
blis·ter pearl | ˈbli-stər ˈpər(-ə)l
noun
An irregularly shaped and hollow pearl, cut from the shell of an oyster
Typical pearls are grown inside the tissue of an oyster. Blister pearls, however, grow against the inside of an oyster’s shell. As a result, blister pearls must be cut out of an oyster. Additionally, rather than being round they are often hemispherical in shape and have a flat back. Though not shaped like a typical pearl, blister pearls are beautiful and very popular in pearl jewelry. They come in a wide array of colors, ranging from light pink to bluish hues. Some of them even have a rainbow-like gleam. In rare instances, blister pearls have a beautiful golden swirl. In order to prepare blister pearls for jewelry placement, special procedures must take place. After it is harvested from its oyster by being cut out of its shell with a circle-bit drill, its nucleus must be removed. Next, the blister pearl’s hole is filled with resin. Finally, the back of the pearl is capped with Mother of Pearl. After Blister pearls are harvested and prepared for jewelry placement, they are called Mabe pearls.
Of course, the experts on this site can offer a much more educated assessment. This is just my guess.
www.pearl-guide.com
