This is a modular necklace. The chain you wear with your gateway is what holds your core in place. To change it out simply remove the chain from the gateway and tip it out. Slide your next core in through the top, run the chain back through and go about your day.
A hypoallergenic black snake chain comes with the gateway. They come in 16, 20, 24 and 27 inch lengths.
Black gateways are made with anodized aluminum.