Which agent can create a shallow layer of asterism in a corundum cabochon?

The presence of titanium oxide is significant in the context of corundum, specifically when discussing the phenomenon of asterism. Asterism occurs when light reflects off needle-like inclusions, commonly rutile, present within the gemstone. In corundum, titanium can contribute to the formation of these inclusions, leading to the characteristic star effect when the stone is cut as a cabochon.

When titanium is incorporated into the crystal structure of corundum, it can form minute needle-like inclusions that, when oriented correctly and viewed under appropriate lighting, produce the desirable star effect associated with asterism. This effect can create a shallow layer of asterism depending on the quantity and arrangement of inclusions present.

In contrast, the other agents listed, such as zinc oxide, bismuth oxide, and iron oxide, do not typically produce the same asterism-inducing inclusions in corundum. While iron oxide can affect the color of corundum and potentially contribute to other visual effects, it does not create the needle-like structures necessary for asterism. This further reinforces why titanium oxide is the correct choice for creating a shallow layer of asterism in a corundum cabochon.