Translation:There are seven diamonds in the piece of jewelry.
Actually, "piece of jewellery" sounds fine to me. You could even leave out the jewellery and say "in the piece", so long as you were aware of the context. Jewellers / antiques people and the like are forever referring to pieces of jewellery as "pieces" ... it's a linguistic pet hate of mine which ranks up there with the affected pronunciation of "theatre" as "thee-ET-er" (hence me being super-aware of it), but it exists.
The additional "piece of" is entirely derived and is throwing the whole translation. DL needs to be less artistic. They should be translations not interpretations