If your jewelry contains platinum, gold or sterling silver you can't describe it as such unless it's hallmarked.
Otherwise, the only formal requirements are those associated with running any business on a self-employed basis. i.e. you're required to register with HMRC and pay tax on your net profits.
If you're already in full-time employment (or part-time employment earning more than �6035pa), your personal allowance will already have been used up and HMRC will want 20% of your net profits. (I'm assuming that your total income doesn't exceed �34,800)
If selling the jewelry is your only source of income, you can make up to �6035pa in net profit, without paying tax (but you're still required to register with HMRC).
Additionally, if your income from the jewelry business exceeds �105 per week, you're obliged to pay national insurance at the self-employed rate. (That applies even if you already pay national insurance in relation to another job).
Chris