Most alexandrite gemstones used for engagementr rings are under one carat, and it is safe to budget 1.500 to 6000 for an 18K gold engagement ring, with 0.3-0.6cts alexandrite in the center and some diamonds around it.
Alexandrite! (Beware lab stones, are dull. The genuine stones are the ones with all the beautiful color facets.) This would be the perfect engagement ring. Are those channel set diamonds? Not if so, add a custom wedding band with channel set diamonds to