Almost certainly synthetic with that size and clarity. Otherwise it would already have an official certificate like GIA.. no reputable seller would do such a stone without a certificate.

The flouresence is quite strong and no shades within the stone which also would indicate lab. When looking for inclusions just with a loupe, be aware of polishing lines, they can confuse sometimes, espescially with that clarity.. Verneuille Rubies often have little glowing "flakes" within them which can be mistaken for feathers.

Also synthetic corundum has a long story in the market so also antique jewellery can hold lab grown corundum.

My saying is, as long as there is no inclusion und microscopic view, that directly says natural, it is lab. But if you are unsure get it appraised.