How is it determined that these stones are from meteors?

Genuine Tektites are natural glass formations caused by meteorite impacts. The heat from an impact can be hot enough to melt silica (sand), fusing it into glass. This "impact glass" is called a Tektite. All Tektites have unique qualities that make it possible to be identified and the Pearl of Fire has a very similar appearance to a specific type of Tektite, however it has not yet been determined if they are actual tektites or a rare type of volcanic glass. Instead, they are part of a unique category called pseudo-tektites, natural glass formations that are nearly identical to Tektites in appearance and composition.

may i request for the biggest/heaviest possible stone, among the ranged group of 20-30grams?

These stones are pre-packaged and ready to ship! Because of this we are unable to select individual stones.

Does it come with certificate

At this time we do not offer a certificate for this item. As soon as we are able to include a certificate we will update the listing. Thank you, we appreciate your inquiry!