Using Sharpies to create nail art is the new trend, and it's not unhealthy to do if you apply a base coat first to prevent staining. To create a Sharpie marble nail look, all you need are a few coats of white nail polish. Then, use a Sharpie to draw lines and gently brush rubbing alcohol on top for that marble effect.
To achieve a wood grain look, apply a couple coats of gray nail polish, then use a Sharpie to draw vertical lines; finish the look by applying rubbing alcohol over the lines. Another trend to try is watercolor nails; you need three colored Sharpies, rubbing alcohol, a small glass (such as a shot glass), and a nail brush.
Begin with a white base coat, then color the bottom of the inside of the glass with a Sharpie, add drops of rubbing alcohol, and use the nail brush to stir. Apply the color to your white nails, then clean out the glass and do the steps again with a different color Sharpie, and finish off the nails with a top coat.