The price of a blue sapphire is primarily driven by Color, Clarity, Cut, and Carat weight. Color is the most significant factor; the most prized hue is a vivid 'Royal Blue' or 'Cornflower Blue' with high saturation and medium-to-dark tone. Clarity in sapphires is judged by the absence of visible inclusions, though 'silk' (fine rutile needles) can sometimes prove natural origin and even enhance value by softening the color. A well-proportioned Cut ensures the stone reflects light evenly without 'extinction' (dark dead spots) or 'windowing' (transparent areas). Finally, Carat weight impacts price exponentially; a 5-carat sapphire is significantly more expensive per carat than a 1-carat stone due to its rarity in nature.