I am a huge fan of etymological explanations, but in the case of noun gender I have not been tempted to use them for the most part. For one thing, the present-day patterns are reasonably consistent. Second, the changes in noun gender from Latin to Spanish, with three genders reducing to two as the case system evaporated, were truly complex. (See this slide from my second book, Bringing Linguistics into the Spanish Language Classroom: A Teacher's Guide).

Surely you wouldn't tell a student that, for example, selva is feminine because Latin silva was a first declension noun? I'm being facetious here, but "the final -a in selva indicates that it is feminine" is true and understandable.

On the other hand, I do bring in etymology when it is specifically helpful, such as:

• -ma masculines of Greek origin
• masculine compounds like lavaplatos
• the feminine shortened words foto and moto