Ontogenie jewelry is cast in solid sterling silver, solid brass and solid bronze as well as gold-plated brass. The silver and bronze pieces, and to a lesser extent brass, will tarnish and discolor slightly over time. This is completely normal for these materials. Since moisture tends to accelerate the tarnishing process, I recommend that you don't wear the jewelry in water, e.g., while bathing, showering or swimming. The oils in your skin will also contribute to tarnishing. I recommend polishing the jewelry with a soft cloth after wear and storage in a dark, dry place to slow the oxidation which causes tarnish.
If the jewelry should tarnish, I've found that it can be returned to its original brilliance and color by using a gentle, nonabrasive silver polish. The more detailed and highly textured pieces will be more difficult to polish because of surface variations. To polish these, I recommend using a very soft toothbrush with the silver polish to allow cleaning of small crevices. After polishing, you can remove excess polish by rinsing the jewelry in water and then washing the piece in with a gentle detergent followed by a final water rinse.
The gold-plated materials, however, should not be vigorously polished as the plating is likely to wear off over time. The gold-plated materials are much less susceptible to tarnishing and will probably never need to be polished.
All of the jewelry should be protected from impact against hard surfaces as this could scratch it.