The letter samekh represents support, protection and memory.
When pronouncing the letters, its first letter is called nigle, revealed, whereas the others are nistar, hidden.
There are four letters whose gematria of the revealed letter equals the gematria of the hidden ones:
These four letters form the word sium, conclusion.
A similar phenomenon takes place in the Tetragrammaton. (source: Gaon from Vilna).
In some legends, samekh is said to have been a miracle of the Ten Commandments.
Exodus 32:15 records that the tablets "were written on both their sides." The Jerusalem Talmud interprets this as meaning that the inscription went through the full thickness of the tablets.
The stone in the center parts of the letter samekh should have fallen out, as it was not connected to the rest of the tablet, but it miraculously remained in place.
The Babylonian Talmud (Shabbat 104a), on the other hand, attributes this instead to samekh, but samekh did not have such a hollow form in the sacred paleo-hebrew alphabet that would presumably have been used for the tablets. (source: Sanhedrin 21b-22a)
Samekh teach us circular thinking, so to be inclusive of everything and everyone. It is the principle that the wisdom is not contained in just one vessel, in just one person, but it is distributed in all beings.
The combination of nun and samekh forms the word nes, miracle. Once we have learned the lessons of these two letters, we can discover what the miracle really is. Below you can see the movie of the production process of a clay prototype of a hebrew letter. Later on the prototype is used to design a jewel.