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I want to create a magma slime, which has a magical vulnerability to being cooled down and turns to rock easily (like when hit by a water or an ice spell). This rock can then be shattered to gather valuables.

Is it possible that quickly cooled magma/lava would form any valuable materials like gems, or would it only form igneous rocks?

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    $\begingroup$ Can you explain the nature of the monster's life? We can explain what science knows about rapidly cooling molten rock (molten metal becomes solid metal, molten silica becomes glass, molten basalt becomes solid basalt but, as Monty explains, there's crystallization and a fine texture), but the nature of what makes the monster alive could and will result in non-science-based results. In other words, if you want gems, those gems could be a result of minerals combined with the life essence of the monster becoming something valuable. That would be a world rule, not science. $\endgroup$
    – JBH
    Dec 1, 2023 at 3:45
  • $\begingroup$ Is there anything magical about your creation, or not? How does vulnerability to being cooled down and turning to rock require magic, rather than being hit by water or simply sitting around and waiting? $\endgroup$ Dec 14, 2023 at 21:37

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Quickly cooled lava form a naturally occurring glass called obsidian

Obsidian is formed from quickly cooled lava, which is the parent material. Extrusive formation of obsidian may occur when felsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome, or when lava cools during sudden contact with water or air. Intrusive formation of obsidian may occur when felsic lava cools along the edges of a dike.

Obsidian is mineral-like, but not a true mineral because, as a glass, it is not crystalline; in addition, its composition is too variable to be classified as a mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color, similar to mafic rocks such as basalt, the composition of obsidian is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% by weight or more. Crystalline rocks with a similar composition include granite and rhyolite. Because obsidian is metastable at the Earth's surface (over time the glass devitrifies, becoming fine-grained mineral crystals), obsidian older than Miocene in age is rare.

About its value:

Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads in a process called knapping. Like all glass and some other naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture. It was also polished to create early mirrors.

A notorious use of obsidian blade was as cutting edge in macuahuitl

The macuahuit was a common weapon used by the Aztec military forces and other cultures of central Mexico. It was noted during the 16th-century Spanish conquest of the region. [...] It was capable of inflicting serious lacerations from the rows of obsidian blades embedded in its sides. These could be knapped into blades or spikes, or into a circular design that looked like scales.

The macuahuitl was sharp enough to decapitate a man. According to an account by Bernal Díaz del Castillo, one of Hernán Cortés's conquistadors, it could even decapitate a horse

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A minor frame challenge: You're not looking for gems that might form as it dies, but gems that it reasonably might have

Diamonds, Peridot, Garnets, I think Rubies and Sapphires as well all form in the mantle, and are brought to the surface by volcanic activity. Your magma elemental could easily do the same.

There's no reason to limit yourself to just "minerals that form from cooling magma" though snowflake obsidian and others can be very cool

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Going by scientific studies of lava cooling, large crystals are formed when lava cools slowly. Slowly cooled lava/magma becomes granite, while quickly cooled lava becomes basalt.

So, a quickly-cooled body of lava would not be expected to form large crystals, much less large, valuable crystals, unless they were already within the body, due to having a higher melting point.

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    $\begingroup$ You might want to specify that "slowly cooling" means millions of years. $\endgroup$ Dec 1, 2023 at 4:18
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Real world example: pillow lava

Pillow lava

(gif from Richard Pyle via USGS, longer and slightly higher-res clip available on internet archive, full documentary "Where Pele meets the Sea" is available for free online if you search)

That's an underwater eruption of lava in the ocean, so it gets cooled extremely quickly, whilst the inside cools slower and remains molten for longer. Might be a good model for the motion of your beastie whilst it is under attack.

The skin of the pillow lava is glassy, because crystal grains don't have time to grow large and the pressure of the extruding magma underneath prevents the formation of actual large chunks of glass because the cooling crust is being continually flexed and heated. Unless your magma monster stays absolutely still whilst you cool it, you're unlikely to get nice macroscopic chunks of glassy stuff like obsidian, but more like chunks of basalt.

Gem formation is a complex topic in itself, but suffice to say you don't find a lot of gems in volcanic eruptions. Stuff like kimberlite (a famously good material for finding diamonds in) forms deep in the mantle under considerable heat and pressure and over geologically significant periods of time, not over a few minutes or hours of conflict on the surface. Of course, animate molten rock monster don't exist in the real world either, so if you want them to be diamond piñatas then no-one can reasonably stop you.

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Would it be possible that quickly cooled magma/lava would form any valuable materials like gems

No, gemstones form in a lot of ways but never by cooling lava. However it may have 'eaten' some adventurers and have some ceramic false teeth floating in it, or gold, or as many gems as you want. It depends if it incorporates gems etc,. into it's body after ingesting/absorbing them. Which is totally up to how you want your story to go.

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While normal slimes are regularly depicted as homogeneous blobs, magma slimes are much more complicated internally. They have several internal organs that filter various materials from their bodies.

For example:

  • Magma slime gizzards use internally formed diamonds (formed from carbon under heat and pressure) to grind up absorbed materials to be further digested.
  • Their gallbladders/livers/kidney equivalents remove the copper, silver, or gold melted from various adventurers coins they have ingested.
  • Certain magical items that are resistant to fire are not destroyed by being absorbed and are instead sealed into cyst like cavities inside the slime.

These hidden treasures aren't obvious when killing the slime as the majority of their bodies do solidify into various rocks, but a good strike with a hammer will reveal the inner treasures. Older larger slimes will naturally have absorbed more material and have more internal treasures.

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