The wonderful little classical building was either a temple or a tomb, and it may have been built in 77, 115 or 175 CE, by either an independent Armenian king or one who had been appointed by the Romans. And that is the relatively certain knowledge about it.
After the Armenians converted to Christianity in the 4th century, they destroyed almost all the pagan monuments in the country. For unknown reasons, this was spared; to my mind this is strong evidence that it was in fact a royal tomb.
The tomb/temple stood until 1679 when it was destroyed in a truly terrible earthquake. This photograph of the ruins was published in 1918.
In the 1960s Armenian archaeologists and politicians got to thinking that since the building had been largely left alone since its collapse, and many of the pieces were still obviously lying around on the site, they could rebuild it. So they did. You can see that for missing pieces they used unmatching stone, so you can tell the difference. Narratives I have found online say it is "almost entirely" original, but staring at photographs I get the impression that a good quarter of the fabric is modern. Since the building was built in the most traditional form for a Greek temple, there was no difficulty sorting out which pieces were what.
But anyway it is a very cool building. I tend to think that more ruined structures should be rebuilt, when there is sufficient knowledge to do so accurately.