Gold filled wire, findings, beads, crucifixes and centers are made with a thin sheet of gold or silver bonded
to an inner core of base metal (often copper or brass). Filled parts, especially gold, are much more expensive than standard plated (that is, electroplated over base metal), but less expensive than solid precious metals.
Gold filled is considered a jewelry grade material, where base plated is not.
Bali vermeil beads and bead caps are made with a heavy (standardized) layer of
18k or 24k gold plated over solid Sterling silver. These are high quality, precious metal parts.
Bronze caps and chain are, in general, an antique plated finish over solid brass or steel.
One exception are Bali cast bronze bead caps and connectors, which are solid bronze. Some chain
and caps are true vintage with natural patina, not plated.
All metals used are specified in each item's detail page, so that you know exactly what you're buying.