Eight years since his international debut are we any closer to discovering what, exactly, Jamie Roberts is made of? Because it surely isn’t just flesh and bone. Granite and gutta-percha, perhaps, or oak, tar, and iron. Next time Roberts is cut take a close look at the wound to see if you can catch a quick glimpse of the adamantium skeleton government scientists must have grafted to his bones when he was a young boy. In the bar at the City Arms on Quay Street, close to the Millennium Stadium, I once heard word that Roberts’ fists are actually made out of the very same metal as Thor’s hammer, Mjolnir, and that they were, in fact, forged by Dwarven blacksmiths on the distant planet of Asgard.

On Saturday Roberts was man of the match, as he should have been when Wales drew against Ireland the week before. In that game he made 21 tackles, one of them a chest-high hit on Robbie Henshaw so brutal that it drew loud boos from thousands of Irish fans inside the Aviva Stadium, despite being entirely legal. Or legal, at least, according to the laws of rugby. Had it been on the street he would have had to answer for assault and battery and grievous bodily harm. Against Scotland he made another nine tackles, one more on his own than Wales’ back three managed between them.

Roberts, 6ft 4in and almost 18 stone, forms a kind of one-man Maginot Line in the Welsh midfield. When you’ve got the ball, the only sensible approach is to try to go round him. And when Roberts has it, I guess you just shut your eyes and say a quick prayer. His game-breaking, victory-shaping try came in the 65th minute, when the Scots were still three points up and doing a good job of defending their own try line. Roberts came in off a five-yard run on an angle slanting back in towards the last tackle, aiming for a narrow gap between the prop WP Nel and flanker John Hardie. It was a rare instance of the mountain coming to the man.

Nel braced himself, and caught Roberts in the gut with his right shoulder. You wonder what was going through Nel’s mind at that moment. And the answer, of course, is Roberts’ right elbow. A split-second after Nel hit Roberts, Hardie came in on him from the other side. By now Roberts’ body was rolling around as Nel disappeared beneath his feet, as though he had thrown himself in front of a racing coach and four. So Hardie made contact with Roberts’ back, and wrapped his left arm around his hips.

The try line was still seven or eight feet away. Nel, who is 19 stone, had a hold of Roberts’ ankles and Hardie, 16 stone, had a hold of his thighs. And Roberts, with 35 stone of Scotland player chained to his legs, just kept on going.

On many days, against most players, Nel and Hardie would have done enough. But from that range Roberts is an irresistible force. It was not one of the great tries. In fact there were four better in this match alone, a fine pair by the Scots, one made by a dinky chip by Finn Russell and the other by Duncan Taylor’s deft sidestep, and another couple by Roberts’ team-mates Gareth Davies and George North, who both sprinted in from long distance. But it was typical of the man, and the way he plays. If Roberts collided with a running bull, the team would be eating beef for a week.

Roberts has been doing this kind of thing for the best part of a decade now. It is almost a surprise to find he is still only 29, and so could carry on doing it for a long while yet. He says he has a third of his career left. Given the way he plays, his body should be in bits.

But Roberts has now played – a remarkable statistic this – in 31 consecutive Six Nations Tests. In the past two years the only matches he has missed have been the soft ones before and during the World Cup, the warmups and group stage game against Uruguay. Despite that load, he seems to be playing as well as ever. He has been rejuvenated, he says, by his move to Harlequins and the two-month spell away from the game studying at Cambridge after the World Cup. Even then, Roberts played in a few fixtures for the university team.

Pity poor Henry Lamont, the 21-year-old Oxford student who had to line up opposite Roberts in the Varsity game before Christmas. Lamont left the field after 23 seconds of the match because he was concussed in their very first collision. Roberts is studying for a masters in medical science, and perhaps this is his secret. He is not only able to follow the old advice and heal himself, but also understand with expert precision how best to inflict wounds on the opposition.

Some say Roberts’ Cro-Magnon style of rugby is one reason why Wales’ back play has sometimes seemed so unreconstructed under Warren Gatland. There was an article in the Welsh press last weekend calling him both “the solution and the problem”. But they would be foolish to wish him away. Everyone else, on the other hand, is doing exactly that.