carbon fiber
Hair-thin black filaments (about 5 to 10 micrometres across, thinner than a human hair) made almost entirely of carbon. Thousands are bundled into a tow and woven or laid straight, then set in resin to make the stiff, light black parts of bikes, aircraft, and racing cars.
Carbon atoms are arranged in graphite-like sheets (strong covalent bonds in the plane) aligned along the fiber axis, which is why the fiber is so stiff and strong lengthwise. Typical modulus is about 230 GPa (high-modulus grades reach 400 to 600 GPa), tensile strength about 3.5 to 6 GPa, and density only about 1.8 g/cm^3, giving specific stiffness far above steel or aluminum. It is made by heating (pyrolysing) a precursor fiber, usually polyacrylonitrile (PAN), and stretching it so the sheets align.
Carbon fiber delivers the best routine strength- and stiffness-to-weight of common structural fibers, which is why it rules aerospace and high-end sport. Honestly, though: it is expensive and energy-intensive to make, brittle (little warning before it snaps), a good electrical conductor (which can cause galvanic corrosion against aluminum), and, as a thermoset composite, very hard to recycle.
A road-bike frame or an airliner tail made of carbon fiber in epoxy is as stiff as its metal predecessor at roughly half the weight; the fibers, not the resin, carry that stiffness.
Superb along the axis, weak across it; the finished part is strongly anisotropic.
The fiber is superb along its axis but weak across it; the finished carbon part gets its across-fiber and shear strength only from the resin and the lay-up, so it is strongly anisotropic, not uniformly super-strong.