Does homeowners insurance cover fireplace explosions?

Unfortunately, questions like this are hard to answer without more information. What I can say is that generally speaking, when a loss is sudden & accidental, it is usually covered by almost all modern homeowner's policies. Instances that a loss like this would not be covered can include failure to properly maintain the fireplace or malicious mischeif (perhaps a homeowner was drunk and tossed a bunch of fireworks into the fireplace).

The most important phrase to remember with homeowner's insurance is "sudden & accidental". If it didn't occur over time and if it didn't occur because you purposefully caused the loss, you generally would have some coverage available.