However, depending on the context, you could use one of these options:
* As "tephra" refers to the material ejected from a volcano, you could use:
* Vulkaanstof (volcanic dust) for fine ash.
* Vulkaanpuin (volcanic debris) for larger pieces.
* Vulkaanuitbarstingsmateriaal (volcanic eruption material) for a general term.
* If you're discussing the geological aspect, you could use:
* Pyroklastiese materiaal (pyroclastic material) which is a more technical term.
Ultimately, the best translation will depend on the specific context and the level of technicality you want to achieve.