This is not an Antique Engagement Ring.  It is an Estate Diamond Engagement Ring or Dress ring with a 2.6ct F/G Diamond set into 18ct White and Yellow Gold. Yes- that is more than two and half carats of nearly colourless Diamonds. The centre stone is a White Gold bezel set 2.58 ct Brilliant Cut Diamond surrounded by a Yellow Gold rim, 10.50 - 9.05mm wide White Gold half round shank with Yellow Gold rounded edges, stamped 750 for 18ct gold.  The ring is in a very good condition and is accompanied by an independent valuation which notes the details of the ring and its stones as follows:

Central Diamond measurment: 8.75 - 8.80mm

Diamond weight: 2.58cts

Colour: F/G Border

Clarity: I1

Weight: 16.90 grams