Moldavite belongs to the gem category known as "tektites"--they are the only known gems to have fallen to earth from the sky. Moldavite--which gets its name from Europe's Moldau River Valley, where it was first discovered--has been used in tools, talismans and jewelry over the millennia. Knowing the properties and history of moldavite can help you distinguish the real thing from a fake.
Examine the stone's color. It should be a dark, almost mossy-colored green. If the stone you're looking at is a brighter, more vibrant shade of green, it may be an emerald instead of moldavite. If the stone you're examining isn't green at all, then it definitely isn't a real moldavite.
Look at the stone's structure--we're talking about the physical composition of the stone. While many gemstones, such as quartz, have a crystalline structure, moldavite does not. In fact, it is classified as having an "amorphous" structure--"amorphous" literally means "without shape," meaning moldavite may grow in several shapes or patterns.
Consider the stone's texture. Because of its amorphous composition, real moldavite is difficult to process into a completely smooth, glassy surface. If the gem in question is the right color, but is as smooth and polished as a piece of bottle glass, it may be a fraud.
Examine the clarity of the stone. Just as it is hard to polish moldavite into a smooth surface, it is also difficult to find a real moldavite that is completely clear throughout. Real moldavite is made of silica, but often studded with impurities such as iron and magnesium that can make it less clear than a stone such as diamond.
Ask the seller where the stone is from. Moldavite, as stated in the introduction, is a tektite found only in Europe's Moldau River Valley, located in what is today the Czech Republic. If the seller claims the stone was mined anywhere but this region, it is almost definitively a fake.
See whether the moldavite easily scratches or dents (warning: this may permanently alter or damage the stone). Real moldavite has a hardness rating of 5.5 on the Mohs hardness scale, the standard scale for measuring gemstones. A diamond has a hardness rating of 10, meaning it's virtually impossible to scratch; comparatively, a real moldavite is vulnerable to scratches and dents and must be handled with care.