American Diamond and Natural Zircon are two different gemstone.
The confusion arises because American Diamond, which is a synthetic (man-made) gemstone is also known as Zircon in trade while already there is a natural gemstone known as Zircon. So, to differentiate between them, it is necessary to add Natural infront of Zircon name.
Therefore, Natural Zircon (http://www.gemstonebuzz.com/zircon) and Zircon (http://www.gemstonebuzz.com/synthetic-cubic-zirconia – also known as AD – American Diamond, CZ – Cubic Zirconia, Synthetic Cubic Zirconia) are completely different gemstone. And, unfortunately, many people in the trade still don’t know about it.
For astrological purpose, it is always advised to wear Natural Gemstones, so it is better to buy Natural Zircon rather than American Diamond.
The eBay link which you have sent is of Synthetic Cubic Zirconia (an affordable alternative to Diamond) which should not be worn for astrological purpose.
However, you can check out this page http://www.ebay.in/sch/i.html?_trksid=p2047675.m570.l1313.TR0.TRC0.H0.Xnatural+zircon&_nkw=natural+zircon for Natural Zircon being sold by many dealers at eBay.in. However, always ask for gemstone certificate which is certified by reputed gemological lab before buying any expensive gemstone (Rs. 5,000 and up), so that you would be certain what you’re getting is real.