Tortoise beads originally referred to amber colored beads with black stripes in them, but now, they reference any translucent bead that has dark stripes in them.
Generally, the stripe is horizontal, but not always. The stripe should be in a parallel pattern versus a random swirl; if the stripe is more swirly, you have a swirl bead and not a tortoise bead.
The blue beads in the middle have a poor tortoise pattern, as they are quite curved and a little random.
Tortoise beads come in a variety of shapes, sizes, and colors. Large tortoise beads make great focal beads for your bead projects.