Graphite is a soft, flaky, crystalline form of carbon commonly used by artists for drawing.
Of the 90 active elements found in nature; nitrogen, oxygen, hydrogen, and carbon compose 96 percent of all matter.
Along with diamond, graphite is an allotrope  of elemental carbon , the two materials being the same but for their structure. Graphite is a mineral, which has metallic properties including thermal and electrical conductivity. It is grey to black, opaque, and has a metallic luster. Not to be confused with metallic lead, graphite is non-toxic and blacker in colour. The mis-connection to lead is possibly due to the use of lead rods by scribes to write on papyrus in ancient Rome and that graphite was originally mistaken for lead until it was discovered to be a unique mineral.
History The name graphite comes from the Greek “graphein”, ‘to write’ and was named by German chemist and mineralogist A. G. Werner in 1789. Graphite came into widespread use following the discovery of a large and remarkably pure graphite deposit in Borrowdale, (Lake District) England in 1564. The discovery and its subsequent use formed the basis of the renowned pencil industry in Keswick.
Development of the Pencil Darker than lead the pure graphite was a wonderful mark making material because the nature of its layered structure lends itself to deposit particles on paper or other rough surfaces when rubbed over them but it was too soft and brittle to make writing with it easy unless it was strengthened, initially wrapped in string  or sandwiched between strips of wood .
Conrad Gesner Illustration of a pencil 1576
The oldest reference to what we now call ‘the pencil’, graphite encased in wood, is an illustration by Swiss naturalist Conrad Gesner  in one of his books, published in 1567. Replicas based on his illustration called the ‘Gesner Pencil’ are still available to artists today  Architect and entrepreneur Joseph Hardtmuth is often credited with inventing the first graphite pencil in Vienna in 1790. In France, the lack of quality graphite sources during the Napoleonic wars led the French chemist Nicholas Jacques Conté to experiment with the poorer quality graphite available in France and in 1795 he patented a new process for making graphite pencil ‘leads’. This method mixed powdered graphite and clay in a water slurry, to form rods or strips, which were hardened in a kiln. These composite graphite-clay rods allowed for more efficient use of graphite and produced a superior writing and drawing material, it revolutionized the pencil industry. Not only did the formula reduce costs, but the ratio of clay to graphite powder allowed more control of the lightness and darkness of the graphite mark left on the paper, a whole range of pencils could be made.
Use by Artists today It is available today in the form of pencils, woodless sketching pencils, studio pencils, blocks, sticks and powder, it is also available, though expensive and hard to find, in its natural lump form.
Artist Pencil An artists pencil consists of round core of graphite encased in wood. The most common of all our drawing instruments the graphite pencil is a versatile drawing tool with a varied range capable of great contrasts, textures and subtleties. The pencils we use today are still made with a mixture of graphite and clay and can be graded by how dark or light they are based on the quantity of clay and graphite. The more graphite used the softer the pencil and darker the mark, the more clay used the harder the pencil and lighter the mark. This grading or measurement has been standardised by hardness (H) and blackness (B), (F) indicates that it can be sharpened to a fine point.
Sketching Pencil or leadless pencil These pencils have no wooden outer core which means there is more surface lead to rub over the paper. Available in a range of graphite density but their weakness is that they will break if dropped on a hard surface.
Graphite Sticks Graphite can be found in sticks or blocks up to 2” wide that I have found. Useful for making quick gestural figure studies, sweeping landscapes or larger drawings. In block form all parts of the graphite can be used, sides, corners, ends which give it more scope to create wonderful marks and textures than a pencil. Blocks and sticks can be found in a variety of grades from hard to soft.
Studio Pencil Similar to a very early pencil and to modern-day carpenters pencil in design the lead is in broad flat strips held between wood. The advantage of these is that they can create a fine or broad line just by turning the pencil round. Coloured Pencils Colored pencils are made from chalk, clay, or wax mixed with binders and pigments.
Graphite Powder Graphite powder is useful for large expanses and softly graded tones. The powder is easily rubbed onto paper with a soft square of cloth. It can be removed or drawn into with a rubber and burnished to create metal like surfaces. It can be slippery to use in powdered form, the structure of graphite is thin layers separated by graphine hence its use in industry as a lubricant.
Clutch Pencils Similar to some of the earliest pencils the clutch carries a bare rod in its barrel and holds it in place for use. A really useful tool for those who carry sketchbooks and draw on the go because the lead is protected inside the body and does not break off like a pencil might in the pocket. Clutch pencils come in a variety of sizes the most common for artists being: 2mm, 5.6mm with a huge variety of rods to fit. 
Some quirky facts about graphite
- At surface temperatures and pressures, Graphite is the stable form of carbon..
- One pencil has the potential to draw a line 35 miles long or write an average of 45,000 words 
- A line drawn by a pencil will conduct electricity. 
- All diamonds at or near the surface of the Earth are currently undergoing a transformation into Graphite
The Pencil: A History of Design and Circumstance, by Henry Petroski. http://apassionforpencils.com/passion/history
Henry Petrowski, The Pencil: A History of Design and Circumstance, New York: Knopff, 1990
James Watrous, The Craft of Old Master Drawings, Madison: University of Wisconsin Press, 1957
Keith Vaughan Journals 1939-1977, ed. Alan Ross, London: John Murray, 1989
Jaspar Salwey The Art of Drawing in Lead Pencil, London: Batsford, 1921
John Ruskin, The Elements of Drawing in Three Letters to Beginners, London: Smith, Elder & Co., 65 Cornhill 1857
Mervyn Peake, Craft of the Lead Pencil, London: A. Wingate, 1946
Barbara Hepworth, Drawings from a Sculptor’s Landscape, with an introduction to the drawings by Alan Bowness, London: Cory, Adams & Mackay, 1966
 Allotropes are compounds that have the same chemical makeup but different structures
 The hardness of minerals is compared using the Mohs Hardness Scale, a relative scale numbered 1 (softest) to 10 (hardest). Graphite is very soft and has a hardness of 1 to 2 on this scale. Diamonds are the hardest known natural substance and have a hardness of 10. http://www.enmu.edu/services/museums/miles-mineral/diamond.shtml
 Sometimes written as Gessner