The function appears to apply the rule consistently to all input words, with the exception of "Zircon", which returned a different result. However, upon further investigation, I found that "Zircon" is a mineral, and the function may have returned "Mineral" because it is a type of mineral that is often associated with the word "Zircon". Therefore, the domain of the function where the rule applies is limited to words that are not minerals.

I hope this helps! Please let me know if you have any further questions or if you would like me to run additional tests.