Sgian Dubh translates from the Gaelic to black dagger, the handle was usually carved from black bog oak. The 'black' also implies 'covert', as this was a weapon of last resort it was concealed and only removed in the presence of a host out of courtesy and usually placed in a sock where it would be seen.
The Highland Dirk has been since the 15th century distinctive from European daggers having a single edge only, the heavier blade perfectly balanced for use also as a short sword. The oldest dirks had carved knotwork pommels, plain scabbard and simple ornamentation. By the 18th century the ornamentation had increased to what we tend to see today.