Hi Hugh, these are notoriously hard to date unless hallmarked. Ive seen quite a few of these over the years and have never been able to say for sure how old they are. The problem is the style has never really changed that much throughout the years. The name Sgian Dubh roughly translated means black knife in gaelic. The small Saint Andrew and cross is usually referred to as a "Jimmy" by many Scottish Collectors, don't ask me why, I have no idea where that comes from, its a popular icon mostly in the Highland Regiments, the Black Watch wear it as a collar badge as well as on the Sgian Dubh. so its hard to pin down your Sgian Dubh to a particular Regiment without some kind of provenance. On the up side, it does look to have quite a bit of age to it, I would agree with Mervyn its most likely late Victorian or possibly early Edwardian. A lot of Victorian "Cairngorms" in Dirks are not true Cairngorms at all, they are actually glass, backed with a foil and coloured to give the impression of being a Cairngorm, as true Cairngorms were considered semi-precious stones and would add significantly to the cost of producing a dirk or Sgian Dubh...so many officers plumped for a faux cairngorm to cut the cost. Unfortunately the only way to find this out is to remove the stone from the pommel. Either way, its still a nice little Sgian Dubh and a nice thing to have.