parshat Tetzaveh: Identifying the Stones: שְׁבוֹ - šəḇo

The second stone on the third row, that of Naftali, is שְׁבוֹ - šəḇo. As Shemot 28:17-21 states:

יז וּמִלֵּאתָ בוֹ מִלֻּאַת אֶבֶן, אַרְבָּעָה טוּרִים אָבֶן: טוּר, אֹדֶם פִּטְדָה וּבָרֶקֶת--הַטּוּר, הָאֶחָד.	17 And thou shalt set in it settings of stones, four rows of stones: a row of carnelian, topaz, and smaragd shall be the first row;
יח וְהַטּוּר, הַשֵּׁנִי--נֹפֶךְ סַפִּיר, וְיָהֲלֹם.	18 and the second row a carbuncle, a sapphire, and an emerald;
יט וְהַטּוּר, הַשְּׁלִישִׁי--לֶשֶׁם שְׁבוֹ, וְאַחְלָמָה.	19 and the third row a jacinth, an agate, and an amethyst;
כ וְהַטּוּר, הָרְבִיעִי--תַּרְשִׁישׁ וְשֹׁהַם, וְיָשְׁפֵה; מְשֻׁבָּצִים זָהָב יִהְיוּ, בְּמִלּוּאֹתָם.	20 and the fourth row a beryl, and an onyx, and a jasper; they shall be inclosed in gold in their settings.
כא וְהָאֲבָנִים תִּהְיֶיןָ עַל-שְׁמֹת בְּנֵי-יִשְׂרָאֵל, שְׁתֵּים עֶשְׂרֵה--עַל-שְׁמֹתָם; פִּתּוּחֵי חוֹתָם, אִישׁ עַל-שְׁמוֹ, תִּהְיֶיןָ, לִשְׁנֵי עָשָׂר שָׁבֶט.	21 And the stones shall be according to the names of the children of Israel, twelve, according to their names; like the engravings of a signet, every one according to his name, they shall be for the twelve tribes.

Thus, JPS translates šəḇo as "agate."
Midrash Rabba states: נפתלי אבאטיס
Jastrow (pg 2) writes that this is a scribal error and the word should be read אכאטיס, with a kaf in place of the bet. One page 62, he states it is the Greek term for "agate."

What is agate?

According to Wikipedia,

Agate is a term applied not to a distinct mineral species, but to an aggregate of various forms of silica, chiefly chalcedony.

Wordnet defines it as

an impure form of quartz consisting of banded chalcedony; used as a gemstone and for making mortars and pestles

According to Easton's 1897 Bible Dictionary, under agate, (cited by dictionary.com) they write:

(Heb. shebo), a precious stone in the breast-plate of the high priest (Ex. 28:19; 39:12), the second in the third row. This may be the agate properly so called, a semi-transparent crystallized quartz, probably brought from Sheba, whence its name. In Isa. 54:12 and Ezek. 27:16, this word is the rendering of the Hebrew cadcod, which means "ruddy," and denotes a variety of minutely crystalline silica more or less in bands of different tints. This word is from the Greek name of a stone found in the river Achates in Sicily.

Here is a picture of some banded agate
but look around for other pictures.