Oxblood
Oxblood as a color is defined as a "dark brownish red", or slightly more broadly as a "dark or deep red to medium reddish brown". It was literally named for the color of the blood of an ox; a common variation of the name is the French "sang de boeuf". It was named during a time when people lived in multi-species communities, and depended very closely on animals to supply many of their needs. They were familiar with their animals to the extent of knowing the different shades of blood of different species. Other shades of red had other names, e.g. sang de poulet (chicken blood), sang de pigeon (pigeon blood), peach bloom, crushed strawberry, crimson, liver, ....
Here are a couple of examples of oxblood I found in non-marble contexts. An "oxblood red" swatch and some glazed ceramics:
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A few more -- another palette example, another glaze example and a much more brown example in dyed wool:
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edit: a little after I gathered those I learned that there is a somewhat official definition of the color "oxblood red". One which has been cited in U.S. patents is Robert Ridgway's
Color Standards and Nomenclature. So in a patent when a rose or an apple was said to be Oxblood Red, someone could consult the swatch on Plate I and know what was meant.
In the context of marble collecting, most of the time the term oxblood refers to certain opaque reddish glass found in Akros, M. F. Christensen bricks (American Cornelians), and some German handmades. MFC bought the American Cornelian formula from James Harvey Leighton, part of a famous family of glass makers and glass chemists. Akro obtained their formula from MFC by dubioius means, hence it would be fair to call MFC the standard. However, time and familiarity have given Akro this distinction.
Here are some Akro examples posted by Lloyd:
The oxblood in those examples is an opaque microcrystalline copper-based glass. The opacity comes from the size of the copper crystals. It is related to aventurine in a fascinating way. See Brian Graham's discussion here for more about this phenomenon,
Melting Oxblood or Haematinum red glass. This glass can take on different shades, and in this era we marble collectors seem to be less precise with the names of the shades, tending to dwell on the chemical nature of the glass rather than the exact color. This makes sense, considering that these days we tend to be more familiar with chemistry labs than with barnyards.
Note: Oxblood-colored glass in this family of copper-based examples is not always opaque. In the 1800's and early 1900's there was a copper-based red glaze which was called sang de boeuf and was transparent.
This type of oxblood -- the Haematinum red glass -- is known to command relatively high prices in comparison with many other versions of red. Sellers with many different motivations are aware of this reality and this sometimes leads them to market things as oxblood which are not Haematinum. It is the buyer's responsibility to be aware of what they are buying and avoid paying Haematinum prices for non-Haematinum. Other versions can be fun and attractive, but be aware of the difference. (note: there is a version of red made with gold -- I know little about that and nothing about its value in comparison with oxblood.)
Now what about that oxblood from other marble makers? We hear of Vitro oxblood, Alley oxblood, horsehair oxblood, Peltier oxblood, etc. A key point there is that people tend to add a word on the front such as "Vitro" or "horsehair" to make it clear they are not necessarily trying to claim that it is the same as that found in classic Akro oxbloods. Some might be copper based, and some might even have been made intentionally, but be careful about assuming too much. Here are some which look like copper-based oxblood. First is a mib Gary posted which was ID-ed as an Alley, then some pelts -- two multicolors of Carole's, one with similar colors from Marblealan, and another fascinating one of Carole's. Kinda looks like oxblood might be an accidental byproduct of the chemicals used to make the turquoise glass, doesn't it?
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[space reserved for more examples, inc. ones called oxblood which don't look all that close to Akro's version]
An interesting occurence of oxblood in recent years has been achieved at Jabo, at first by accident, and now on purpose. It was discovered when aventurine cullet was fed into the furnace during a tank wash late in 2007, but it was oxblood which came out, not aventurine after all. In earlier years some Jabo classics had a different type of red which some call oxblood; that is selenium-based glass.
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