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A kind of rock · also called scoria, lava stone, volcanic rock
About
Lava rock is a porous volcanic stone that is surprisingly light because trapped gas bubbles fill its volume.
Lava rock forms when gas-rich volcanic lava cools quickly into hard stone riddled with air pockets. Because of these voids, its bulk density is only about 47 pounds per cubic foot (0.75 grams per milliliter). This makes a pile of lava rock roughly half as heavy as standard gravel or crushed granite of the same volume. Its lightweight nature allows it to take up significant space without adding excessive weight in garden beds and gas grills.
From Wikipedia
Volcanic rocks are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks.
Read more on Wikipedia →Text from Wikipedia’s “Volcanic rock” article, CC BY-SA 4.0.
One cup weighs
6.26 oz (177 g)
Quick answer
One US cup of lava rock weighs about 6.2 ounces (177 grams). That is about the same weight as a cup of granulated sugar, despite being solid stone.
Typical figures, the same ones every comparison on the site uses; real ones vary.
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Next to anything: a person, a car, another rock.