Viola tricolor var. arvensis|
Viola bicolor Pursh
One quirk of the violet is its elusive scent - a major component of the scent is a ketone compound called ionone, which temporarily desensitises the receptors in the nose; sniff all you like, you won't get any more smell from the flower!
In North America, there are several different species of wild violets. Some are blue, some are yellow, white, or cream; some are even bicolored. In addition, the shape of the petals defines more species; for example, some violets have a "spur" on the end of each petal. The violet is the state flower of Rhode Island.
The common commercial African violets are usually a deep purple (though they can also produce pink flowers), and are derived from a few specimens collected in South Africa; the species is threatened in the wild.
Australia is home to a number of small 'native' violets. These have low-lying clusters of green foliage and very small white to pale purple flowers.