the.com/diamond knot

means A decorative, ball-shaped knot (also called a Lanyard Knot) tied in rope to form a stopper or ornament, often doubled and tightened into a faceted knob that won't slip through a hole or pull loose.

from From its gem-like look: once tied and hauled taut, the interwoven strands bunch into a faceted, many-sided lump that catches the light like a cut stone, so sailors named it for the jewel it resembled rather than for any stone-cutting craft behind it. It comes out of maritime rigging and marlinspike seamanship, where it served as a 'lanyard knot' — a solid stopper worked into the end of a line so it couldn't slip through a deadeye or block. The 'diamond' is purely descriptive, not a borrowing from another language or trade.