Extending the lifespan of materials, textiles and apparel is a main strategy to move towards a circular economy and one of the pathways to achieve the reduction of material carbon emissions. To date, materials circularity approaches for bio-derived circular materials mostly focus on recycling or biodegradability. Enabling new properties of circular materials to emerge would provide not only alternatives to renewable materials but a way to encourage material longevity through creative design-led approaches. This paper presents the Circular Materials Design Toolkit (CMDT), a tool for the textile and apparel industry to explore possibilities for material longevity by creatively differentiating their products using circular materials technologies and processes developed in the UKRI Interdisciplinary Textiles Circularity Centre (TCC). The Toolkit emerged from interdisciplinary collaboration in TCC materials circularity research and includes the Materials Library cards which communicate the known technical specification of materials and processes to investigate circular design possibilities. Three studies were conducted with apparel brands and material start-ups using the CMDT to understand, develop and apply circular materials and processes in the context of circular value chains and process flows. The studies uncovered how the Toolkit enables new material properties, practices, and process flows to achieve longevity in textiles circularity.