On stoikheion, the second gloss here, see eta 614.
The Greek language lost the sound of s in initial position and between vowels, but various processes resulted in new sibilant sounds (Smyth, sections 103ff., 118ff.: web address 1 & web address 2).
The letter sigma comes from the Semitic sin (Hebrew שׂ), one of four letters representing sibilants in the Canaanite alphabet. Of the others, zayin (Hebrew ז) became Greek zeta, samek (Hebrew ס) gave xi; and sade (Hebrew צ) was used in some alphabets instead of sigma, but survived eventually only as a numeral with the value 900.