Why did Jesus use pearls in his teachings?

The four final questions (all designed to enable an answer to the main question) can perhaps be best dealt with by dealing first with any "cultural significance" in the era Christ spoke of pearls.

Pearls were certainly known to be beautiful, rare, and valuable to all people back then. Even the poorest person would have heard of pearls, if not seen them adorning kings and queens as they processed before the populace. In 1 Timothy 2:9 Christian women were advised not to deck themselves with costly pearls, or gold. People back then likely envied merchants who gained great financial wealth by trading in pearls, as well as High Society people who flaunted their pearls and gold.

It may have sounded strange to them, Jesus giving a parable about a man who sold everything he owned in order to purchase "a pearl of great value", but maybe there were literal pearls worth far more than what that man had, so to invest in such a rare pearl would be understood by them.

But all those cultural matters were only aids to grasp the spiritual truths Jesus taught. And grasping this reveals why Jesus chose not to use gold, but used pearls. So, why did Jesus use 'pearls' for spiritual truths in those two teachings, instead of gold?

In the first text, Jesus said not to cast their pearls before pigs. Now, his audience were unlikely to have people who owned even one pearl. But they knew about pigs even though they were forbidden in God's law to keep pigs or to eat their meat. They, like us, know the damage pigs do to ground where they forage for food, turning it to mud if they are confined to too small an area. They could imagine the incongruity of a person coming along with a handful of pearls to throw on the ground in front of pigs. They would know that pigs just scoff up what is presented as food, and pearls would be swallowed down (though gold could not), or trampled into the mud. Then, if the pigs were unsatisfied, they could well turn on that person. Further, any pearls swallowed would come out through the alimentary canal, re-appearing in the vilest form. The verbal picture Jesus presented would be crystal-clear; things of great value and worth are not to be presented to those who would trample them (and those doing the presenting.)

That is why Jesus spoke in parables. That is why those two particular parables taught about entering into the kingdom of God, if but the listeners knew it, for mention of pearls signified this entrance into the kingdom of God. Jesus heeded his own warning, and spoke in parables, but those to whom he gave "pearls of wisdom" were enabled to "enter in".

"But where shall wisdom be found? and where is the place of understanding? ...The gold and the crystal cannot equal it... No mention shall be made of coral, or of pearls: for the price of wisdom is above rubies... Seeing it is hid from the eyes of the living, and kept close from the fowls of the air." Job 28:12-21 extracts A.V.

Christ is the Wisdom of God. Find Christ, and you have found the pearl of greatest price - priceless, actually. Everything material is worth giving up so that the wisdom of God be found in Christ (1 Corinthians 1:20-31).

Now, the second saying might be better understood by appreciating something about the hidden, secret development of a pearl within a living oyster. Here I quote:

“It is wrongly supposed, traditionally, that a particle of sand begins the interior process whereby something is formulated within the fleshy tissue inside the shell of that creature, deep beneath the sea. What is grown is a composite of living secretion and of structural content, laid down, layer by layer, in laminate form until the end result is harder than bone. Its tensile strength is akin, I understand, to tooth enamel. It is of living substance, but it is structured, note well.

But what actually begins all this formulation is neither granular nor particulate – it is living. Either bacterial or viral in form, it is an infection that prompts the oyster to respond in the way it does. Thus, beneath the swelling waves of the sea… within the outer shell of the animal and, deeper still, in the very depths of the creature is another life… This life is that which causes a pearl to be produced – in time – within the depths of the oyster. And this – this – is the way in to the city Jerusalem which descends from God out of heaven [the kingdom of heaven]. For the pearly gates which men talk about and write about are not gates made out of millions of pearls that they might form a gatelike structure of mere pearly material. Not at all.

The way into the city is by a gate – which is a pearl. Each several gate – is a pearl. The pearl – as such, considered – is the way in to the heavenly city. The entrance is a matter of a life that, becoming resident deep within the creature, causes a structure to be formed that is more durable than flesh, and stronger than bone.

This shall endure unto everlasting life. For it is Christ himself who is formed within, in Spirit, when the Gospel is imbibed by the soul. But it must be the true Gospel – the apostolic Gospel, delivered in the beginning by Jesus Christ himself through chosen apostles, and conveyed, subsequently, by those called of Christ himself and from the throne of glory in the heavens.” The Gates of Pearl, pp48-49, Nigel Johnstone, Belmont Publications, 2016

Jesus used pearls to teach the only way into the kingdom of heaven. Do not try to enter by scrabbling about for either trampled down pearls or those dropped by those who are spiritually outside of the kingdom of heaven. Go only to Christ, the Wisdom of God, and his words of life. Those who have already entered into the kingdom of God freely share pearls of wisdom Christ gave freely to them. They have diligently searched, finding Christ to be the Way, the Truth and the Life. Priceless!

First let's address "How did first-century culture view and use pearls?" This turns out to be a very interesting question indeed. Pearls are mentioned nowhere in the Hebrew Bible except the Book of Job, and the word translated as "pearls" has an uncertain meaning. Interestingly the pearls known to the Israelites were probably red, not white. According to the Jewish Encyclopedia:

It is doubtful if pearls are mentioned in the Bible. Usually, one of the two words "peninim" and "ra'mot" is taken to mean pearls. Both are objects of great costliness (comp. Job xxviii. 18). In Lam. iv. 7 "peninim" is supposed by some authorities to indicate objects whose color is red-—probably red pearls.

This brings us to what Jesus meant when he spoke the word (in Hebrew or Aramaic) that Matthew recorded as "pearl." Would it be a red pearl or a white one? It could be either, for by this time white pearls would surely have been known through the trade with Mediterranean. Interestingly, the J.E. article indicates that some ancient rabbis thought pearls were found in fishes. This adds another cultural dimension to the question probably not envisioned in the OP. An ancient Jewish story tells of a tailor who finds a pearl inside a fish, which might be related to the NT story of the disciples finding a valuable coin in the mouth of a fish. (Matthew 17:27)

More to the point, rabbinical tradition holds that a pearl can be a metaphor to denote any valuable teaching or interpretation (Ḥag. 3a). This is relevant to Jesus' parable, where the pearl is exactly that - being the gospel in Mt. 7 and his teaching regarding the Kingdom in Mt. 13.

Having dealt with the cultural aspects, let us turn to the other specifics:

• Why a pearl rather than gold? In Mt. 7, because gold would not lose its value by being trampled, but pearls would become worthless by being crushed. In Mt. 13, because a huge pearl is worth much more than a piece of gold of the same weight. (Pearls can be worth as much as $100 million in today's dollars.)

• Culturally, would a single pearl warrant selling everything? Certainly yes, based on the above-mentioned valuation. In terms of Roman culture rather than Jewish, pearls were the ultimate status symbol. Julius Ceasar even decreed that they could be worn only by royalty. Wearing a fabulous pearl was tantamount to being a king or queen.

But of course, Jesus was speaking of a "pearl of wisdom" that involved the attainment of eternal life. He used the pearl, rather than gold, as a symbol because it was far more valuable than gold, carried the implication of royalty (perhaps relating to being a adopted as a son/daughter of God) and - in the case of Mt. 7 - was also fragile, and should not be exposed to rough treatment by those who did not value it.