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Thanks for the comment Masamune. You are correct that certain materials do make the crystals appear much more easily, and that cooling slower will allow bigger crystal grains to form. I work for a materials analysis company (x-ray diffraction). Practically all metals when cooled will naturally form crystals, and these crystal grains are in various sizes, orientations, and shapes in the metal. Steels typically produce very small crystal grains when cooled, where as aluminum and lighter metals produce bigger grains. The trick to making the grains stick out when etching these materials with acids is to find the correct acid and strength, and immediately clean and oil the etched location immeadiately. if you want to do this, however you require acids and safe working practices. be careful! here is a website that has lists of metolographic etchants you can try. (aluminum is shown.) http://www.metallographic.com/Etchants/Aluminum%20etchants.htm