brooch; chest ornament
A decorative item worn on the chest, typically a brooch or pin. The term is straightforward and neutral.
祖母からもらった胸飾りをドレスに付けた。
I attached the brooch I received from my grandmother to my dress.
Compound of 胸 (mune, 'chest') and 飾り (kazari, 'decoration'). The exact historical derivation is uncertain, but the word transparently means 'chest decoration'.