a single crystal
Imagine one unbroken tile pattern covering an entire floor with no seams anywhere: every tile lines up with the same grid, from one wall to the other. A single crystal is like that. Every atom belongs to one continuous, uninterrupted lattice that runs across the whole piece of material.
In a single crystal the periodic array is unbroken throughout the entire sample — there are no grain boundaries dividing it into misoriented patches. Single crystals are grown deliberately by slow, carefully controlled solidification: the Czochralski method pulls a giant silicon boule from a melt, and directional solidification grows turbine blades. The silicon wafer at the heart of every computer chip is sliced from one enormous single crystal.
Two features make single crystals special. They are strongly anisotropic, with clean, well-defined direction-dependent properties. And, crucially for jet engines, they have no grain boundaries to slide and creep along at high temperature — so the hottest turbine blades are grown as single crystals to survive the extreme stress of spinning while red-hot. Most ordinary engineering metals, by contrast, are not single crystals.
A modern jet-engine turbine blade is grown as one single crystal of a nickel superalloy. With no grain boundaries to slide along, it resists creep at the extreme temperatures where an ordinary polycrystalline blade would slowly stretch and fail.
Single-crystal turbine blades: no grain boundaries, so far less high-temperature creep.
A single crystal can show a beautiful flat-faced habit, but external faceting is not required; what defines it is one unbroken lattice, not its shape. Most engineering metals are NOT single crystals.