The main factors are color (hue, tone, saturation), clarity, cut quality, carat weight, and rarity/origin. For most colored gems, color dominates valuation—vivid, pure hues with medium tone and high saturation are most desirable. Clarity grading considers type: Type I gems (e.g., aquamarine) are expected to be eye-clean; Type III (emerald) tolerate visible inclusions.
Cut affects perceived color and brilliance. Larger sizes are rarer per carat, increasing price non-linearly. Geographic origin, confirmed by lab reports, adds premiums for certain deposits (e.g., Burmese ruby, Colombian emerald). Treatment status is factored into all comparisons.