Tektites 

You will receive one piece of tektite, the total weight of the piece will be at least the amount quoted in the variation.

Tektites (from Greek τεκτός tektos, molten) are natural glass rocks up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular.
 
Tektites are among the "driest" rocks, with an average water content of 0.005%. This is very unusual, as most if not all of the craters where tektites may have formed were underwater before impact. Also, partially melted zircons have been discovered inside a handful of tektites. This, along with the water content, suggests that the tektites were formed under phenomenal temperature and pressure not normally found on the surface of the Earth.
The terrestrial-impact theory states that a meteorite impact melts material from the Earth's surface and catapults it up to several hundred kilometers away from the impact site, which means that it must have travelled through space (thus explaining the dryness). The molten material cools and solidifies to glass. According to this theory, a meteorite impact causes their formation, but the precursor material of tektites is primarily of terrestrial origin, as determined from isotopic measurements. Today, the terrestrial origin of tektites is widely accepted based on the results of many geochemical and isotopic studies, e.g. Faul, H.(1966) and Koeberl, C.(1990).
 
The impact theory relies on the observation that tektites cannot be found in most places on Earth's surface. They are only found in four strewnfields, three of which are associated with known impact craters. Only the largest and geologically youngest tektite deposit in Southeast Asia, called the Australasian strewnfield, has not been definitively linked to an impact site, probably because even very large impact structures are often not easy to detect. For example, since the Chesapeake Bay impact crater (today the largest known impact structure of the United States and associated with the North American tektite strewnfield) is covered by sediments, it was not detected until the early 1990s. Also, the bigger the strewnfield, the bigger the area to search for the crater. Since several new craters are identified every year, this is not really regarded as a problem by proponents of the tektite impact theory, except for the expected Australasian crater, a feature that would be less than a million years old and thus easily visible. This crater, if it exists at all, has not been located, but there are some candidates
The ages of tektites from the four strewnfields have been determined using radiometric dating methods. The age of moldavites, a type of tektite found in Czech Republic, was determined to be 14 million years, which agrees well with the age determined for the Nördlinger Ries crater (a few hundred kilometers away in Germany) by radiometric dating of Suevite (an impact breccia found at the crater). Similar agreements exist between tektites from the North American strewnfield and the Chesapeake Bay impact crater and between tektites from the Ivory Coast strewnfield and the Lake Bosumtwi-Crater.
 
Below are some types of tektites, grouped according to the four known strewnfields, and their associated craters:
 European strewnfield (Nördlinger Ries, Germany, age: 15 million years): Moldavites (Czech Republic, green)
 
Australasian strewnfield (no associated crater identified; but see Wilkes Land crater and above references): Australites (Australia, dark, mostly black)
 Indochinites (South East Asia, dark, mostly black)
 Chinites (China, black)
 
North American strewnfield (Chesapeake Bay impact crater, USA, age: 34 million years): Bediasites (USA, Texas, black)
 Georgiaites (USA, Georgia, green)
 
Ivory Coast strewnfield (Lake Bosumtwi Crater, Ghana, age: 1 million years): Ivorites (Ivory Coast, black) 

Each piece has it's own unique shape and internal markings, for this reason the item received may differ slightly from those shown in the photograph.