What organic material displays a structure called "engine turning"?

The correct answer is associated with coral, specifically concerning the unique patterns that can be found on certain types of coral. Engine turning is a decorative technique that involves creating intricate geometric patterns, which can be seen in some coral varieties that exhibit a natural or enhanced engraving resembling this method. This technique often enhances the visual appeal of coral jewelry and art pieces by adding depth and texture to their surfaces.

Paua shell, on the other hand, is known for its vibrant colors and iridescence but does not typically exhibit engine turning. Similarly, conchiolin is a protein found in shells and does not relate directly to any machining or decorative carvings. Ivory, while it can be carved to display detailed designs, does not possess the natural patterns that signify engine turning as clearly as coral can in some contexts. Thus, coral stands out as the relevant organic material associated with this specific decorative technique.