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04 | Create a Reflection
Select the REFLECTION layer and make it visible. Select the circle and fill it with a gray color (R:204 G:204 B:204. Make a copy of this circle (Cmd/Ctrl + C or Edit > Copy) but don't paste it anywhere just yet.
Double-click the Scale Tool in the toolbox and enter a value of 94% in the Uniform input box in the dialog box that appears. Click OK.
Paste the copy, made moments ago, in front using Cmd/Ctrl + F. For the sake of contrast, fill this circle with electric green (R:102 G:255 B:0).
Move the green circle to the upper-left a bit until you can see a sliver of the gray circle beneath it. For reference, see the image below. If necessary, hide the layers below to see things more clearly.
Select both circles (Cmd/Ctrl + A) and open the Pathfinder palette (Window > Pathfinder). Press and hold the Alt/Option key as you click the Subtract from shape area command. This command uses the green circle to cut away from the gray circle below leaving only a tiny sliver of gray.
Note: Older versions of Illustrator use the Minus Front command on the Pathfinder palette instead of the Subtract from shape area command and do not require you to use the Alt/Option modifier key.
Select the sliver and fill it with white (R:255 G:255 B:255). On the Transparency palette, set the blending mode to Overlay and lower the opacity to 30%.
Lock this layer, select GRADIENT and make it visible.
05 | Gradients Ahoy!
Select the circle on this layer, double-click the Scale Tool in the toolbox and enter a value of 80% in the Uniform input box in the dialog box that appears. Click OK.
Apply a gradient fill to the circle and adjust the settings and values in the Gradient palette (Window > Gradient) as shown below:
On the Transparency palette, set the blending mode to Screen and lower the opacity to 80%.
Using either the Selection Tool or the arrow keys on your keyboard, move the gradient circle to the upper-left interior of the original circle (on the COLOR layer). Leave a little breathing room so the edges are not touching. Use the image below as a reference. Lock this layer and continue.