Candlelight

On the 'Tables of Elipses' exercise, the degree of the elipses is not always consistent, so it is important to make sure each in a row have the same alignment.

Also on the 'Rotated Boxes' exercise, the boxes are a little too cube-ish (they are meant to be tapered boxes) , and they are slightly wonky, getting more skewed around the outside. In future make sure the boxes taper and use the neighbouring edges of the boxes to estimate the position of the other edges that are soon to be drawn.

In my opinion I think you should do what you want. Reading Lesson 0 in regards to the 50% basically says to draw things you want, even when you lack the skill.

Watching and following tutorials is a good idea (I have been looking at some portrait tutorials), but I would not stress too much. I don't know much about jewllery, but I found some tutorials on YouTube:

https://m.youtube.com/watch?v=Szw9ki8UAZ4

https://m.youtube.com/watch?v=67A8uRFU920 (she starts at the 1:15 mark)

Maybe the sphere could feature a highlight on it? Also the cube could use its own cast shadow, as it looks a little flat.

For my criticism on the three sections (firstly on the Funnels exercise), make sure to put the short major axis in, as it is not present in the drawing. Some of the lines on the Rough Perspective exercise also appear to be drawn multiple times, when they are only meant to be drawn in one, consistent stroke, so make sure to only do the one stroke (though you still can use ghosting).

Furthermore in the Rotated Boxes exercise the center box is a little too thin and needs to be more squarish, with the height and width equal to each other.

On the funnels excercise, make sure the sides of the funnels are of equal distance. Some of the funnels are not fully equal, leading to the elipses not being divided fairly in half.

Also, some of the boxes on the rotated boxes excercise are a little wonky, specifically around the back of the boxes. In future I would be careful to make the lines parallel to the horizon line.

Finally I would add that there is not enough distortion of the boxes on the very outer edges. I would try to make them taper more.