Carnacite is a mineral substance, pink in color, which is highly toxic in its natural state. Processed carnacite is used in construction as filler material.
Bleak Prime was a major source of carnacite; the planet's carnacite deposits were so rich, its entire atmosphere was made unbreathable.
When the Dominion took over Bleak Prime during the Dominion War, it was discovered that carnacite also had the effect of weakening the Founders' control over the Vorta. The Vorta who had been stationed to Bleak Prime also conspired to use carnacite on the Jem'Hadar, and cause a rebellion against the Founders. When this plot was discovered, the Dominion abandoned their carnacite mining on Bleak Prime. (DS9 short story: "Change of Heart")