Umbrite

Pure umbrite is essentially the darklight analogue of adamantine, but colloquially (and historically, before it was truly isolated), umbrite refers to the crystalwood-soak material. This is similar to how people often refer to carbon fiber/epoxy composites as "carbon fiber".
Visually, 'true' umbrite takes the form of a black body. It can also be physically manipulated with some effort (can actually plastically deform instead of just winking from existence), somewhat related to darklight's relatively slow movement speed. Umbrite is a perfect insulator, completely unaffected by radiation of any kind. When shattered, dissipates into fading darkness.
Umbrite-infused materials generally take purplish-black hues.
Created by super-compressing Darklight , suspending it in a Crystalwood matrix not unlike creating fiber-reinforced composites. The toughest materials would be formed from turning crystalwood into paper, mimicking the modern fiber lay-up method. However, it was more practical (especially for complex shapes) to carve a chunk of crystalwood into shape and simply soak that, although the resulting piece would be significantly weaker (think glass micro fibers vs fiber layup). The resulting materials had similar mechanical properties to bronze (plus its famous perfect insulation), although high-quality layups could compare to steel in tensile strength.
Abyss dwellers, not having the same easy access to crystalwood, mimicked the process imperfectly with the Shattered Jungle's trees. The resulting material was less than ideal, although easier to produce.
Those that cannot see darklight cannot comprehend its shape, instead only seeing a black imprint on the world. This makes it especially useful in weapons, as it messes with depth perception.
Within the Darklit Cities, umbrite formed an odd barter good, eventually becoming a part of the penal system. Many large cities had standardized cut blanks meant to be filled with magic to create umbrite for use in construction and crafts.

Umbrite Matrices & Composites

Pure umbrite is created in a vacuum as opposed to suspending darklight within some kind of matrix. Generally speaking, umbrite-infused materials gain mass due to the process naturally pushing other substances (generally air) into a superimposed state at their position: this is crucial to keeping the darklight matrix stable. While some substances are more efficient than others at creating stability (and consequently result in less added mass), there is no way to get around adding significant amounts of mass to gain umbrite reinforcement. Pure umbrite is simply compressed darklight, created in a vacuum as to have no medium to require superimposition.
Essentially, the roles of resin and fiber are reversed compared to real-world fiber reinforced composites: the 'resin' (darklight) actually provides strength, while the 'fiber' (crystalwood, plus the superimposed materials, generally air) actually provides structure.
Darklight naturally does this weird shit where it reflects radiation (especially on the visual spectrum) but visually appears to be pure black (implying it absorbs everything) due to dimensional fuckery (reflecting said radiation into another dimension?). Umbrite concentrates the darklight in an area enough to have it perfectly reflect all radiation (including thermal), keeping said darklight in place through the medium (typically crystalwood).
Crystalwood is the perfect medium because it's naturally extradimensional with links to the plane of darkness, adapted towards soaking up the stuff anyway. Other wood will sorta work, but is nowhere near as efficient due to not having this property (other than smokewood, which would work fine but grows in a region where the settlers are super averse to darkness). The organic nature of the material is important to soaking in the darklight- metals etc. don't soak it in no matter how hard you try.
Interestingly, this means that you could theoretically make super-umbrite by soaking spidertooth fiber from bonespiders. There's no reason that one couldn't, but the amount of energy required would be orders of magnitude higher than crystalwood due to the higher mechanical properties giving a higher magical 'specific heat', plus spidertooth fiber is ludicrously expensive. It's really hard to justify pouring multiple human souls' worth of energy into a few bolts of spidertooth cloth formed from a hundred slain beasts just to see if anything would even happen. Unless you're a certain Shrouded.

Umbrite & Darklight nonclemature

Some refer to darklight in general as umbrite. It's technically correct, sorta like calling water 'melted ice', though admittedly a lot more elegant than its analogy.