Is 0.68mm too thin for everyday wear?

It is on the delicate side. It holds up for normal wear and small pendants, but it can kink or stretch if tugged, so it is not the chain for rough daily use or heavy charms.

What does diamond cut mean on a chain?

The links are cut with small facets that reflect light. That makes a thin chain look brighter and more noticeable than a smooth cable chain of the same width.

Can I put a pendant on it?

Yes, but keep the pendant small and light. A heavy pendant puts strain on 0.68mm links and the spring ring clasp.

What is a spring ring clasp?

A small round clasp with a spring-loaded opening. It sits close to the neck and is common on fine chains, though some people find it fiddly to open.

How should I care for a 14K gold chain this thin?

Store it flat or hanging so it does not tangle, and keep it away from perfume and chlorine. Wipe it with a soft cloth rather than pulling on the links.