Material capital is any durable good that is used as a factor of production and, by virtue of being durable, it is gradually consumed in production over a maximum possible duration of a length that is determined by (i) how much a unit of capital is used and (ii) the depreciation rate of a unit of capital. Capital forms by labor and savings (which is in accordance with the labor theory of value) and/or capital forms by chance (which is in accordance with the marginal theory of value).
Imagine, for example, a solitary person that is stranded on a tropical island. To stay alive, that person must eat and, on the particular island, imagine that virtually every bit of edible biomass is a coconut. In order to be able to consume any coconuts, coconuts must be produced. Without any tools, this person must climb a palm tree to obtain a coconut and, once back on the ground, then must find a way to crack it open. If the person has obtained enough coconuts to last long enough for the person to take a day off from climbing trees then the person, by virtue of their coconut savings, can afford to leisure. Yet, perhaps the person decides not to leisure and, rather, decides to produce tools (i.e. to labor) to make coconut production easier. Perhaps the person fashions a pole out of bamboo to knock coconuts from the tree so that the person doesn't have to keep climbing trees, which reduces the amount of work required to produce coconuts and, therein, makes coconut production that much more productive by virtue of the bamboo tool, which is a form of material capital (as it can be used repeatedly to produce coconuts, which cannot be used repeatedly). As Abraham Lincoln (1861) once said, "Labor is prior to, and independent of, capital. Capital is only the fruit of labor, and could never have existed if labor had not first existed." Yet, capital also forms by chance. To break open the coconuts, the person might use a rock and, perhaps, if there were no rocks on the island that could be used to crack open coconuts then, thanks to misfortune, the person might starve.
Capital cannot be discussed without also discussing tools and technology; they are inseparable. A tool is when capital and a technology coalesce. For example, a makeshift wooden spear is a hunting tool. It is a coalescence of wooden material (capital) and a hunting technology. The wooden object of which the spear is made is material capital, which depreciates at a rate at which the wooden material’s maximum strength decreases. Knowledge about how a wooden object can be used as a spear is a hunting technology. Such technical knowhow is cognitively embodied (in some form or another) in the living body of the tool user, and it is expressed in the body’s behavior, which is behavior that determines how the material object is used (and effectively renders it a tool). This theory has support in the life sciences. (Alfred Marshall  once wrote, “Economics [is] biology broadly interpreted.”)
The phenotype of an organism extends beyond the individual organism, as expressions of an individuated organism’s genes are not contained within that individual organism. Gene expression can be manifest in the behavior of its kith and even in the behavior of another species entirely. “Genes in orchids express themselves phenotypically [as] changes in bee behavior, which result in [a] transference of pollen grains containing those same genes” (Dawkins 1984). The extended phenotype of any individuated organism is the gene expression that extends beyond the embodied physiology of that individual, and it is “less purely biochemical than . . . the conventional local phenotype” (Dawkins 1984). Among the many kinds of phenomena that can be considered to be of an extended phenotype are artifacts like fabricated shelters and tools. “Caddis larvae live in houses which they themselves build out of stones [or] other material. The form of the house is determined by the behavior of the builder, which is presumably influenced by the builder’s genes” (Dawkins 1984). The genome of a builder is the energetic scaffolding of the behavior of the builder (Hoffmeyer 2014), and such behavior, which is gene expression, determines the form of what the builder builds.
The coalescence of capital and a technology as a tool is a process of semiosis. A tool is a sign of capital signifying a technology, which is a notion that comes to be more clear in reference to Ferdinand de Saussure’s semiological conception of the (dyadic) sign. The makeshift wooden spear is a sign of capital signifying a technology. The wooden spear is a hunting tool, and a tool is a sign. The wood of which the spear is made is material capital, and capital is a signifier. The knowledge about how the wooden object can be used as a spear is a hunting technology, and a technology is a signified. The wooden spear, which is a hunting tool, is a sign of wooden material capital signifying a hunting technology, and the arbitrariness of the signifier speaks to the old adage that anything can be used as a weapon. Tools are to technology are to capital as signs are to signifieds are to signifiers.
Returning to the topic of capital, material capital extends from human capital (e.g. knowledge, skills, & innovation) and financial capital extends from material capital. Financial capital is part of financial infrastructure, which is a tool by which other tools (and material capital commodities) can be obtained by supplying one's saved income (or positive profit in production) rather than one's labor. Human capital is phenotypic, which means there is a genetic component. As Ronald A. Fisher once wrote, "In organisms of all kinds the young are launched upon their careers endowed with a certain amount of biological capital derived from their parents." (Fisher 1930). Fisher considered the inheriting of household wealth by offspring from their parents to be an extension of a more generalized inheritance process stemming from the inheriting of genes by the offspring from the parents, which is a notion that foreshadowed the Dawkinsian theory of the extended phenotype. Fisher was a protégé of the political economist Leonard Darwin, a son of Charles Robert Darwin. Charles Robert Darwin notably employed the Malthusian population principle from classical political economics (see Malthus 1798) as the architecture for designing his theory of natural selection (see C. Darwin 1860) and, as Karl Marx (1867) once remarked, "Darwin has interested us in the history of nature's technology, i.e., in the formation of the organs [that] serve as instruments of production for sustaining life." Indeed, Charles Robert Darwin's grandfather, Erasmus Darwin, had written of an "Economy of Vegetation" (1791) and, prior to that, Carolus Linnaeus described all life as an "Economy of Nature" (1749). Indeed, "The ultimate subject matter of biology and economics is
one, viz., the life process" (Daly 1968).
NOTE: In answering this question, I've borrowed heavily from two draft works of mine that are on my Academia.edu page (i.e. I have not plagiarized whatsoever): https://independent.academia.edu/AaronHRubin
The following two links are to the two works of mine from which I'd borrowed: