The answer is: Pearl

Rev 21:21

The twelve gates of the New Jerusalem, crafted from pearl, offer a beautiful metaphor for the grandeur and purity of this celestial city. Imagine each gate, not just shining in luxury but representing a richness that goes beyond mere material wealth. Pearls form through hardship: an irritant enters an oyster, leading to layers of nacre that encase that discomfort, resulting in something stunning and valued. This idea mirrors the human experience, where trials can yield growth and transcendence.

Furthermore, each gate represents one of the twelve tribes of Israel (Revelation 21:12), reinforcing the idea that this new community encompasses the fullness of God's people. The notion of twelve in the Bible is rich, indicating completeness and divine order, like the twelve apostles or twelve directly linked celestial beings. Plus, the streets of the New Jerusalem are made of pure gold, reflecting a divine promise of an economy where material condition ceases to dictate social worth.

Symbolically, such opulence isn’t meant to encourage a fixation on material things; instead, it gestures toward spiritual richness—a reminder that what truly enriches our lives transcends the physical realm. An interesting connection is the Golden Rule (Matthew 7:12); just as these grand gates welcome all, the golden principle calls for compassion and good deed-manship among all beings, hinting that love and kindness could grant anyone ‘entrance’ to this heavenly abode. Isn't that a comforting thought?