Earth Science question about rocks?

Question

Stone arrowheads are made by chipping smooth, curved pieces from a rock to for a sharply pointed triangle with razor-sharp edges. Which rock, obsidian or gabbro, would make a better arrowhead? Why?

Answer 1

Most sharp rock implements such as arrow heads, Axe heads, scrappers, spear points, etc are made from very fine grained stone usually flint which is is usually chert or obsidian which is volcanic in origin .... these rocks have virtually no grain and are very homogeneous and are also very siliceous ... they chip well and are very sharp
and make very successful tools for all purposes... Gabbro an ineous rock w/little or no quartz is much coarser grained and will not be able to be chipped to a sharp edge as the samples are showing

Answer 2

Obsidian would make a much better arrowhead than gabbro.

Gabbro is an intrusive rock (like a granite, except gabbro is composed of mostly mafic minerals) that has large crystals which are obviously visible with the naked eye. This would make it fairly useless as an arrowhead as you want an arrowhead to be smooth and sharp.

Obsidian is actually volcanic glass which cooled so fast that crystals did not form (obviously meaning it is an extrusive rock). This lack of a crystal structure makes it possible to make an arrow out of obsidian that is extremely sharp compared to pretty much any other rock on the planet.

Answer 3

Obsidian. Just guessing here, because when obsidian chips, it leaves some very sharp edges. Gabbro is smoother, less glass-like.

Answer 4

Obsidian

Answer 5

Obsidian.

Obsidian is silica that came from a volcano and cooled so quickly that crystalline structure could not form. In the absence of crystalline structure the rock, when chipped, will fracture in concoidal pattern (this is what lets you develop the sharp edge).

Answer 6

Obsidian due to the conchoidal fracture it has.

Answer 7

obsidian , now the location is important.

Answer 8

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