Hi Detlef,
I'm afraid it's as Jose said, a low cost item generally purchased as an accessory to Highland dress worn at weddings etc., they are even made these days without blades or with wooden blades for dress purposes. Better ones are still made with silver fittings, bog oak or rare wood hilts and semi-precious stones and quality steel blades which are very much useable, see
www.castlekeep.co.uk and
www.knifemaker.co.uk for examples of modern hand made types. Sgian dhu to look out for would be 19thC and early 20thC military knives which are assignable to a specific regiments, these can be quite expensive and are highly collectable. The origin of these knives is somewhat debatable. Sgian means knife and dhu means black i.e. 'black knife' not black in colour but black as in hidden. The knife was hidden out of sight on ones person but upon entering anothers house, to show no ill will or intent, the knife would be removed from concealment and placed in the top of the sock thereby showing that the visitor carried no concealed weapon and thereby wished no harm upon his host. Women also carried knives concealed in their skirts or hidden in their clothes under the armpit, these were known as sgian aichles or armpit knives.

After the 1745 rebellion the carrying of weapons was banned in Scotland and these small utility knives, I would suggest, took on a greater importance as a symbol of 'Scottishness' bolstered by Victoria and Albert's Scottish obsession which has carried on to this day. The law here states that you cannot carry a fixed blade knife or a lockback knife but only a folding knife with a blade three inches or less and that you require to have good reason, and I mean a really good reason, even for this. A fixed blade knife with a blade of no more than 4 inches is allowed in the field for hunting e.g. stalking, although I would suspect this is rather elastic where stalking on a private estate is concerned. The exception is the Sgian Dhu as long as you are wearing the kilt, I was informed by a large police sergeant, is allowed and with sharpened steel if you want. I was also told, in the dim distant past, by person or persons unknown that when wearing kilt etc that it was legal to wear a sword and dirk as well as a sgian

very handy down Sauchiehall street on a Saturday night

Hope this has been of some help, if there is any more I can help you with just let me know.
My Regards,
Norman.