There are 13 colors and 14 symbols in the gate-naming conventions. I'm not sure if the "staggered" labels for color (Gold, Platinum, Jade, etc.) and symbol (Knight, Rook, etc.) were intentional, but there are a couple of materials that could balance out the number.

Steel (Similar to iron, perhaps with the colors flip-flopped)
Neon (Bright, glowing color like pink on a black background)
Cobalt (Silver symbol on blue background)
Silicon (Dark yellow symbol on dark green background - close to Jade and may cause some copyright issues if the Knight symbol came up)
Pearl (Similar to platinum, but with a yellowish tint to it)
Wood/Wooden (Not a precious metal, but could work - light brown symbol on a dark brown background)

These were just some ideas. I'm not sure how things are designed on the back end as far as if these were meant to be off by one so they could cycle through 182 gates without a repeat. If so, forget this. If not, I hope I'll be exploring the Pearl Bishop gate someday.