After Long Silence — Sheri S. Tepper

Humankind has colonized the planet Jubal, but only to a point. It is the massive crystal Presences which seem to hold true dominion. In some they inspire awe, in others dread. All that is known for certain is that they restrict travel and technology--and that they can be deadly. It is suspected that the crystals are both alive and intelligent. And before those who would do so can simply destroy them, the Presences must be convinced to break their long silence.