Flies from the Amber

From the back cover:

FORTY LIGHT YEARS FROM EARTH -- the colonists on the world of Unua have somehow managed to keep civilization struggling on, despite twice daily earthquakes and technology lagging far behind that of the mother planet. But now an Unuan mining expedition has discovered an alien mineral they name centrokist -- a stone of incomparable beauty. Yet when the Earth scientists at last arrive to investigate, they find a phenomenon eclipsing the centrokist crystals. For this double-sunned solar system is nestled right next to a black hole. And waiting for them on the verge of the black hole's event horizon is an alien artifact that can't possibly exist!