I don't know exactly, but it is probably a very magical rune letter.
The topic title confuses me slightly, but in maths sigma is considered a summation sign.
Are you confusing Sigma, a Greek character, with the Sigel rune, part of the Norse Futhark? In its original language, Sigel meant the Sun, and held great importance to the Norse people. The Sigel rune was used by Nazi Germany, where it was called the Sig rune.
Answer
I'm going to go out on a limb and guess that you do indeed mean the 'Sig' rune, since as pointed out above, Sigma is a greek character and therefore, not a rune.
There are a variety of runes; the primitive alphabets coming from northern Europe, generally carved or painted on stone (no paper at that point in European history). They generally have approximately 22 phonetic characters (which means only one or two, if ANY vowels), and "sig" was the character which phonetically represented s. Its meaning in divination can vary dramatically; ranging from a lightning bolt (which was the germanic interpretation, hence its adoption by the Nazi party) to a snake to a man. Since nobody from that time period still exists, we make our best guesses, but there's no universal 100% interpretation.
F2P: Air rune Earth rune Fire rune Water rune Body rune Chaos rune Cosmic rune Death rune Law rune Mind rune Nature rune P2P: Dust rune Lava rune Mist rune Mud rune Smoke rune Steam rune Astral rune Blood rune Soul rune Extra: Rune Essence(F2P) Pure Essence(P2P) As you can see there is quite a few -Dd_rulez
Fire rune - 0554 Water rune - 0555 Air rune - 0556 Earth rune - 0557 Mind rune - 0558 Body rune - 0559 Death rune - 0560 Nature rune - 0561 Chaos rune - 0562 Law rune - 0563 Cosmic rune - 0564 Blood rune - 0565 Soul rune - 0566 Steam rune - 4694 Mist rune - 4695 Dust rune - 4696 Smoke rune - 4697 Mud rune - 4698 Lava rune - 4699
Sigma Sigma Sigma was created on 1898-04-20.
Rune
Rune berserker no question. Rune Spike is worst, 2nd is rune kite, and best is rune berskerker.
The armor(rune). But there is no magic rune of protection.
Sigma Sigma Rho was created in 1998.
Sigma Sigma Sigma
Sigma Mu Sigma was created on 1921-03-25.
Kappa Sigma Sigma's motto is 'A kappa sig nunquam mendums'.
Sigma Pi Sigma was created on 1921-12-11.
The motto of Phi Sigma Phi is 'Do unto others as you would'.