Chocolate is made from the plant Theobroma cacao's beans. The genus Theobroma of the indigenous jungle tree was so named by Swedish scientist Carl Linnaeus in 1753 for 'divine food' in contrast to gold which was valued mainly for aesthetics by the Aztec civilization; and Montezuma's coffers were found to be overflowing with cacao beans following the conquest of the Aztec Zapotec empire.
Eventually, the Dutch chemist Van Houten invented a hydraulic press following its discovery by Europeans to remove the fat from cacao beans; and the resulting product came to be known as cocoa powder. Van Houten also found that processing this powder with alkaline salts increased its miscibility, making it more effectively mix with water but also reducing its health benefits and flavor. When Swiss chemist Henri Nestle discovered a process for evaporating milk, cocoa was cooked into bar chocolate diluted with milk.1
Cacao is commonly grown in fields neighboring roadsides in Nigeria where they still use leaded gasoline and happens to be an excellent sequesterer of lead, and significant concentrations of lead can be found in cacao produced in Nigeria. Fortunately, there are a lot of cocoa products with cocoa not grown in Nigeria. You should always be sure, and organic brands usually specify the country of origin. Organic chocolate has often 5x the psychoactive chemicals of inorganic chocolate, but you can find suitable brands of baking chocolate online.
Cacao contains little caffeine but much theobromine, a vasodilator and cerebral circulatory stimulant. Cacao is also high in focus and mood-boosting compounds including phenylethylamine which is also concentrated in blue-green algae2. Cacao is similarly an exogenous source of the endogenous immunomodulatory cannabinoid anandamide but in much lower concentrations than endogenous synthesis; however, cacao is much more potent than the body's natural synthesis of anandamide because it has chemicals in it that make it stick around longer.3 While there is little research on the effects of these individual chemicals, studies show that eating chocolate increases performance on standardized cognitive tests.4For a source besides concentrated protein, cacao is very high in tryptophan, the amino acid our bodies make serotonin from. It is a great source of minerals including calcium, magnesium, potassium, zinc, chromium5, and iron6. Raw cacao powder is an excellent source of vitamin C and, according to the Brunswick Laboratories, has the highest ORAC score for measuring anti-oxidants of any food7. The fats in cacao beans are mostly saturated fats.8
1http://www.cracked.com/article_18616_5-bitter-truths-about-chocolate.html
2http://www.livestrong.com/article/204701-phenylethylamine-in-the-diet/
3http://www.chocolate.org/
4http://www.upi.com/Science_News/2006/05/25/Chocolate-increases-cognitive-performance/UPI-90921148615897/, http://www.ncbi.nlm.nih.gov/pubmed/21324330, http://www.ncbi.nlm.nih.gov/pubmed/19942640
5http://www.livestrong.com/article/462015-benefits-of-cocoa-nibs/
6http://www.livestrong.com/article/323705-nutrients-in-cocoa-powder/
7http://www.naturalnews.com/022610.html
8http://www.livestrong.com/article/294723-what-is-the-vegetable-fat-contained-in-cacao-beans/