Elise gives an excellent answer to this question. H Samuel have excellent training opportunities so you can learn in detail about the exquisite products you are selling.
Dress professionally and formally for this interview, and play on your confidence when talking to new people and customers. Working with jewellery, you will be expected to have an excellent patter and strong sales technique, although much of this will be developed over time.
I wish you every success in your interview, but remember, just relax. Be yourself at your best.