The stems, dust and buds separated out during finishing: kukicha (stem tea, also called bocha or karigane), konacha (fine dust) and mecha (rolled buds). Cheaper than the main leaf, but not lesser — each has its own character. The tea served at a sushi counter is usually konacha.
In more detail
Stem tea is savory with little astringency; stems separated from gyokuro or high-grade sencha are often sold as karigane — chiefly a Kansai usage — and can cost more than the leaf they came from. Dust tea gives up its flavor fast and strong, which is exactly why a sushi counter puts it straight in the cup and pours boiling water on it. Bud tea brews dense.