This stone is for educational purposes. It is a flame fusion man-made sapphire but heated and made to "fool" the experts. Mixed with a parcel of stones it appears to imitate the genuine article and is almost indistinguishable. The maker is obviously an expert and has oriented the stone in a way "curved striae" was not detected easily with a loupe despite the size.
This stone is a reminder that if the stone is extremely clean and you can't find any tell tale signs of naturalness, send it to a lab for verification.
AIGS report No. GF18040407 Education stone