As I was scrolling through these textures, I cannot help but feel a little (or a lot) awed by the sheer amount of patience and care that went into the execution of each of these. Some of those on the second page - the yarn and tree cross-section for instance - are just so meticulously and carefully crafted that it really blows me away.

Now, it's one thing to be impressed by beautiful imagery, but I'll do my best to focus on what the challenge/exercise itself asks for, and we'll see where your work could be improved.

Texture is ultimately all about two things - understanding the relationships between different forms as they exist together, usually in a fairly small scale, and the design of the shadow shapes which convey those relationships without the use of explicit outlines. So drawing texture successfully requires us to go beyond simply capturing a repeating pattern, or to draw strictly what we see in a reference - it requires us to understand how those forms exist in the world, and how they relate to one another. I feel that for the most part, you've done an excellent job of this, but I can see some little hiccups here and there that are worth discussing.

The first thing I noticed is present in both your leaf and yarn textures, and it really comes down to the fact that as you tackled these textures, you were still focusing a fair bit on your understanding of objects as separate entities. The yarn is probably the best example of this - every thread is regarded as being its own distinct thing, and so each one receives its own outline separating itself from the other threads around it.

If however we consider the fact that we're dealing strictly in cast shadows - and the fact that those cast shadows are dependent on just how much light we apply (with there being a ton of light on the far right of the gradient, and no light at all on the far left) - then the minuscule barrier between one thread and another, and the shadow it might cast, is so insignificant that it's likely to disappear at the slightest illumination.

It may be a little hard to see, but here I've gone over your yarn gradient a little, knocking out some of the lines that separate threads, and bulking up the cast shadow shapes around those "intersections". As shown in the little diagram to the side, focus your shadow shapes specifically on where the forms meet one another. You can also see this example in the lesson 3 leaves exercise instructions (specifically step 4 where I add the veiny texture). Notice how I'm not actually outlining my veins - I'm adding shadows specifically where they branch out from one another. Lines themselves don't lend themselves too well to being "lost and found" in the way that we require as we get into the sparser end of the gradients. Instead, we rely on cast shadow shapes which can be a lot more dynamic, as shown here. Lines will want to maintain a more uniform thickness, whereas filled shapes can go from very thick to needle thin with no problem.

We run into similar issues with the stones in number 9, where that focus on outlining your textural form makes it quite difficult to transition from one level of density to another. Conversely, the tree bark in number 7 was very well done, and transitioned easily. The difference being, tree bark isn't made up of different pieces to which you can assign a name. Stones, on the other hand, are in your mind separate entities, and therefore you're more prone to drawing them as being separate. At the end of the day, everything we deal with here forms a singular cohesive texture - so whether it's bark where no one piece can be distinguished clearly from another, or whether you've paved a street with fish, you need to think of it all as being part of the same thing. They're all just forms protruding from and adhering to a surface.

Now, there is one other issue that can arise, even when dealing with something that is as contiguous as tree bark, and we see it come up in the various tire textures you tackled for 16, 17, and 18. From what I can see here, you didn't actually capture cast shadows - instead, you ended up filling in the side planes of these tire tread "chunks", in an attempt to distinguish the sides from the tops. This is actually a common mistake I see in the 25 wheel challenge, and the difference is illustrated here. On the left, we end up with a white face along the top that ends up feeling kind of flat, because it's separated from the sides. On the right, we end up with a more complex silhouette with corners that imply the distinction between top and side planes, even though there are no actual internal edges.

Always remember - the cast shadow shapes are always distinct from the forms themselves. They're based on the relationship between a given form and the surfaces around it, but are never as simple as "filling in" one of the form's faces, or filling in a groove.

Anyway! Your work throughout this challenge has been fantastic, so you've certainly earned the badge that comes with it. I'll go ahead and mark it as complete. Keep up the great work!