Final answer:
The leather-hard stage is when clay is dark in color, firm, and stable enough to carve or trim easily, but not completely dry.
Step-by-step explanation:
The state when clay holds moisture and is still dark in color yet is no longer plastic, is firm and stable in form, and can be easily carved or trimmed is known as the 'leather-hard' stage of the clay. At this stage, the clay has been shaped and has begun drying but is not yet completely dry. In the leather-hard state, it is ideal for carving, trimming, and joining because it maintains its shape while allowing for detailed work.