Why gates of pearl?

The foundations of the wall of the city were adorned with all kinds of precious stones: the first foundation was jasper, the second sapphire, the third chalcedony, the fourth emerald, the fifth sardonyx, the sixth sardius, the seventh chrysolite, the eighth beryl, the ninth topaz, the tenth chrysoprase, the eleventh jacinth, and the twelfth amethyst. The twelve gates were twelve pearls: each individual gate was of one pearl. And the street of the city was pure gold, like transparent glass. 

Revelation 21:19-21

I think there is beautiful imagery in the creation of the pearl that will have great significance for those first steps each overcoming believer will take into the holy city of God, the New Jerusalem (Rev. 21). Many Scriptures abound which show that this earthly age is one of warfare for us. God will teach us to do battle in His strength, He will be our hiding place, and He will be our resting place all throughout this journey - but we are engaged in spiritual warfare. This is why the Lord Jesus has called us to be overcomers (Rev. 2:7, 11, 17, 26; 3:5, 12, 21).

Suffering is a non-negotiable part of the Christian life. It has a specific purpose in refining us and works for us a "far more exceeding and eternal weight of glory" (2 Cor. 4:17).

Yes, and all who desire to live godly in Christ Jesus will suffer persecution.
2 Timothy 3:12

And when they had preached the gospel to that city and made many disciples, they returned...strengthening the souls of the disciples, exhorting them to continue in the faith, and saying, “We must through many tribulations enter the kingdom of God.”
Acts 14:21-22

The oyster's way of overcoming the abrasion caused by any grain of sand, pebble or other foreign matter that gets into its soft body is to secrete nacre which cloaks and covers that irritant. This is also how the finest pearls of gemstone quality are formed. The pearl is the product of a process of tribulation. I believe this has something to do with why the 12 gates of the holy city, the New Jerusalem, are gates of pearl. I believe those gates of pearl will testify forever that all the redeemed within have entered by faithfully overcoming tribulation through their faith in Christ. A flat cut diamond or emerald may have well made a sturdy gate, but He chose pearls. I have no Scripture that reveals why God specifically chose pearls for gates, although there will be the abundant use of gold and precious gems of various types in the heavenly city. But since “we must through many tribulations enter the kingdom of God” I think it is worth considering that the God who cares for us, and is taking note of every trial and test we undergo, and who bottles our tears in this age (Psalm 56:8) will set up a pearl as an ornament and entrance into the City where He will give us eternal rest in Himself.

And I heard a loud voice from heaven saying,
“Behold, the tabernacle of God is with men, and He will dwell with them, and they shall be His people. God Himself will be with them and be their God. And God will wipe away every tear from their eyes; there shall be no more death, nor sorrow, nor crying. There shall be no more pain, for the former things have passed away.”
Then He who sat on the throne said, “Behold, I make all things new.”
And He said to me, “Write, for these words are true and faithful.”
Revelation 21:3-5