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The rigid mantle, which is hard and lies above the asthenosphere, is separated from the "crust" that we can see and touch by the Mohorovičić discontinuity. Wikipedia provides an image of what may once have been part of the discontinuity, but I haven't been able to find a good source on what the actual rigid mantle looks like.

Do we know what the rigid mantle itself looks like or how it would look to the naked eye? That is, if I could somehow use my Unobtainium 9000 SuperDrill to drill down to the mantle and excavate a 1 meter squared cube of rigid mantle, what would it actually look like once I got it up to the surface? Would it just look like a slab of random silicates like as could be found in some mine today? Would it look distinctly "alien"?

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  • $\begingroup$ I think it is simple stone, some thousands grad hot and in some thousands bar pressure. This is far lesser for anything hard to imagine. You won't see any very extra-terrestrial - it will glow with very strong, white light. Imagine a cube of white glowing iron, only its light will be about 20 times stronger. Note, this stone will evaporate or at least on room pressure - its solidity is possible only on that pressure. $\endgroup$ – peterh - Reinstate Monica Feb 7 at 17:46
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Above the asthenosphere is the lithosphere. The main constituent of the lower lithosphere is peridotite, a coarse-grained igneous rock containing the minerals olivine and pyroxene. Samples of peridotite can be found in the surface. Sections of ocean crust can be uplifted onto continental margins, forming ophiolites (perhaps the best-known example being the Troodos Ophiolite). They are also found as fragments in basalts and kimberlites, the vertical intrusions that often contain diamonds. So there are lots of places on the surface where you can go and see what the upper mantle looks like! But in the meantime, a closeup of a peridotite is shown below.

Closeup of peridotite from Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Peridotite_closeup.jpg)

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To complete the first answer with bigger scale images, here are some pictures of well-known mantle outcrops in Italy and Corsica (figure 4 from Rampone et al. 2020):

mantle outcrops

The caption reads:

(a) Centimeter-thick pyroxenite layers embedded in the External Ligurides mantle peridotites (Northern Apennines). (b) Partially dissolved pyroxenite layers (substituted by olivine) in reactive spinel peridotites (Erro-Tobbio, Ligurian Alps). (c) Replacive dunite (including pyroxene relics) grading to harzburgite (Lanzo, Western Alps). (d) Spinel peridotite grading to impregnated plagioclase-bearing peridotite (Mt. Maggiore, Alpine Corsica). (e) Two parallel layers of partially dissolved spinel pyroxenites (substituted by olivine) embedded in spinel peridotites. Both peridotite and associate pyroxenites display subsequent plagioclase enrichment related to melt impregnation (see text for more explanation). (f) Plagioclase-rich iso-oriented veinlets in impregnated peridotite.

You can find similar pictures in Piccardo et al. 2007, or in Piccardo 2010.

The Wikipedia article "Peridotite" given in the other answer has some details about those different lithologies:

  • Dunite: more than 90% olivine, typically with Mg/Fe ratio of about 9:1.
  • Wehrlite: mostly composed of olivine plus clinopyroxene.
  • Harzburgite: mostly composed of olivine plus orthopyroxene, and relatively low proportions of basaltic ingredients (because garnet and clinopyroxene are minor).
  • Lherzolite: most common form of peridotite, mostly composed of olivine, orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene). Partial fusion of lherzolite and extraction of the melt fraction can leave a solid residue of harzburgite.

To simplify, lherzolite is a "fertile" peridotite, i.e. a peridotite that can (partially) melts. When melt is extracted, it becomes a harzburgite and eventually a dunite, which are "depleted", refractory peridotites.

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