The phrase "rare jewel" is used to describe something or someone that is very unique, valuable, and hard to find. It's like comparing that thing or person to a precious and scarce gemstone that stands out from the rest. Full definition
Just as someone goes on a treasure hunt to collect rare jewels and treasures, I went on a happiness hunt collecting external things to fill my happiness jar. (inc.com)
«Rare jewels of the forest can now be caught, boxed and shipped almost overnight just like any other express commodity.» (treehugger.com)
I always thought Revelation 21:10 - 11 imaged this place as the slave might have: «And in the Spirit he carried me away to a great, high mountain, and showed me the holy city Jerusalem coming down out of heaven from God, having the glory of God, its radiance like a most rare jewel, like a jasper, clear as crystal.» (religion-online.org)