A year ago I bought one and I just got it is NDC (Nuclear Disarmament Campaign). I don't mind but I wanted feeling peace through everything what I need to having it. And soon I'm gonna practicing vipassana meditation. Can I wear pendant that I have ?
If Mr/Mrs user73898 is doing meditaion seriously in one stream, its would be out of any benefit, if it is done without taking the 8 precepts. Beautification is one Sila of it. Its not proper and every fellow yogi, next to the reality would just think "Oh dear, how mindful... how serious".
If you find a mediation group that has no propel with that, or does not urge Mr/Mrs., run away, all they are doing is for their livelihood and without compassion for Mr/Mrs, without any benefit. Withdraw from sensual pleasure and outwardly notions of conceit (Sila), are prerequisite of meditaion (Samadhi). Not done the raw stuff, how would you waste time in grinding the tree in the forest for getting a nice chair.
(Note: this answer has not been given with the agreement to be means of trade or the purpose of/for trade and/or keep people trapped and bound. How you handle it lies in your sphere, but does not excuse the deed here either.)
For practising any other branch except that of Mahayana / Vajrayana, no restrictions for mundane practitioners' outfit exist, to the best of my knowledge.
However, if you are practicing in any Vajrayana lineage, you should ask your lama, because bearing a visible jewelery / amulet could be a potential obstacle to keeping layman'v vows on Mahayana Sojong days.