When a clay is rock?

In general, clays were considered as a soft material.

It’s true, clay is soft like gels when it is saturated with water; however, clay is a hard rock when it is dried. Look at the photo from Wyo-Ben’s iconic bentonite deposit in Wyoming, the layer of bentonite deposit (dark color), over tens of thousands of years, it still standed and stretched with other rocks above the ground.

Overall, a clay could occur in one of the followwing different states - rock (dehydrated block), paste (water or liquid content is lower than liquid limit), slurry/sludge (saturated with water, oil or other liquids), colloid (suspended in water or other liquid), powder (processed or wind eroded), aerogel (suspended in a gas), a component of soil, carrier of a composite or compound, and so on. Generally, a clay is considering as a matter mainly consists of clay minerals.

In ceramic industry, sometime, clay represents the paste or slurry made from a mixature of fledspar, earth and quartz to produce green ceramics of a porcelain.