We know that Queen Victoria wore the Koh-i-noor in a tiara at least once as she recorded in her journal that, during her State Visit to France, she wore "...my big diamond diadem with the Koh-i-noor in it" for the visit to the Hotel de Ville on 23 August 1855.

As a memento of the visit, Napoleon III sent her a number of watercolours depicting various aspects of the visit. These can be seen in an online exhibition at the Royal Collection site. https://www.rct.uk/collection/themes/exhibitions/queen-victoria-in-paris/bowes-museum-barnard-castle/explore-the-exhibition Unfortunately, none of the paintings depicting Queen Victoria have sufficient detail to gauge what diadem she was wearing, although paintings which show her back indicate that she was wearing a circlet.

The Koh-i-noor itself weighs very little, only 21.6 grams which, for those who use ounces, is just over three quarters of an ounce. https://www.costerdiamonds.com/inside-coster/how-we-cut-the-koh-i-noor-for-the-queen-of-england/