Quick answer
Not always. Chubb, one of the carriers we place valuables coverage with most, only requires a formal appraisal for individual fine art pieces above $500,000, for example. Below carrier-specific thresholds, a detailed description and documented estimated value is often enough to schedule an item.
That said, a real appraisal almost always produces a more accurate value and a smoother claim. We cover what actually counts as an insurance-grade appraisal, and when one is genuinely required, in our appraisal requirements guide.