Although it appears like a grain, quinoa is actually the seed of a plant related to spinach. Since it grows from flowering plant, the blessing is “Ha’adamah:”

בָּרוּךְ אַתָּה יְ‑יָ אֱ‑לֹהֵינוּ מֶלֶךְ הָעוֹלָם בּוֹרֵא פְּרִי הָאֲדָמָה

Bah-rookh ah-tah ah-doh-noy, eh-loh-hay-noo meh-lehkh hah-oh-lahm boh-ray peh-ree hah-ah-dah-mah

Blessed are You, L‑rd our G‑d, King of the universe, Who creates the fruit of the earth.

The after-blessing for quinoa is Borei Nefashot.