In designing a symbol for a company, I needed to create a circle with a perfectly square diamond shape inside of it. All four points of the diamond had to line up perfectly with the circle. I could do this with effects by creating a duplicate of the shape and using Convert to Shape to create a square and then rotate it 45°. But in this case, working with the original vector is easier and faster than effects.
Draw a perfect circle with a black stroke by clicking and dragging with the Ellipse tool while holding down the Shift key. Copy the circle, Command + C (Control + C on PC), then Paste in Front, Command + F (Control + F on PC). With this new front circle still selected, choose the Convert Anchor Point tool (Shift + C) which is hidden under your Pen tool in the Tools palette. Now simply click the four Anchor Points of your top circle converting all the smooth anchor points into corner points and the circle becomes a diamond. Who says it takes a million years to make a diamond?
Tip provided by Jeff Witchel, Certified Adobe® Training Provider.