The Howling Stones

The newly discovered planet of Senisran is a veritable paradise, its oceans dotted with thousands of lush islands containing vast deposits of rare-earths and minerals. But Senisran is also the Humanx Commonwealth's problem child, for each island is inhabited by a different tribe of aboriginal natives. Each has to be negotiated with separately for mining rights - and the Commonwealth is locked in a race against the vicious AAnn Empire to secure those rights.
The clans of the Parramat Archipelago on Senisran are resisting entreaties by the Commonwealth and AAnn alike. But Pulickel Tomochelor, xenologist and first-contact specialist, is confident of his ability to handle to negotiations. What Pulickel hasn't counted on is the secret of Parramat: the strange green stones that the natives use to bless the crops, ensure plentiful fishing, heal the injured and ill, and control the weather.
For within those stones lies an awesome technology the origin of which is lost in time - a technology that has to be kept from the AAnn at any cost. The Humanx Commonwealth: Book Six.