Priority International — package held in mumbai parcels customs

Usps tracking indicates package is being held in customs since 1/23. No updates. What can i do to get it through customs and delivered to addressee or, if prohibited, returned to me?

Ec[protected] u. S.
Contains mineral specimens for gemological institute of india testing, certification and then return to me.

Label describes contents as "merchandise" but these specimens are intended for laboratory testing only (Not for purchase) and will then be returned to me. I want to know how to determine why this parcel is being "held".