When selecting slate for engraving coasters, fine-grained slate is the best choice. This type of slate has a smoother surface, which allows for more precise engraving and helps create clearer, sharper designs. Fine-grained slate is typically less porous, reducing the risk of any ink or engraving material seeping into the stone, which can affect the overall appearance of the final product.

Additionally, black slate is highly recommended for its contrasting color, which enhances the visibility of engraved designs. The dark background allows light colors of paint or filler used in the engraving to stand out, making your coasters visually striking.

Beyond surface quality and color, consider the thickness of the slate. A thickness of around 0.25 inches (6 mm) is ideal, providing durability while still being manageable for coasters. Lastly, ensure the slate is sourced from reliable suppliers who guarantee quality, as inconsistent quality can lead to issues during the engraving process.