Nothing in the institution of marriage requires a diamond. The requirement was, in large part, invented by advertising within living memory. If your intended knows this and delights in it, the alternatives are not compromises; they are statements with better price tags.
The aristocrat
Sapphire is the traditional second choice with royal references to spare, hard enough for daily wear for a lifetime of reasonable care. The blues are canonical, but sapphires arrive in nearly every color, including a pink-orange the trade prices like a sunset. Rubies are the same mineral in red and carry themselves accordingly.
The pragmatist
Moissanite is its own crystal, nearly as hard as diamond and more dispersive, which means more rainbow fire, a tell that readers of gemstones notice and most humans simply enjoy. It costs a small fraction of any diamond and has become the quiet favorite of couples who would rather fund a house deposit than a tradition.
Emeralds, opals, and pearls are beautiful and soft. For a ring worn daily for decades, softness is a maintenance plan.
The fine print
Colored stones are graded by eye and reputation rather than a tidy universal report, so the seller's trustworthiness does more work. Lab-created sapphires and rubies are chemically genuine and considerably cheaper, the same bargain the lab-grown diamond offers. Whatever the stone, the secrecy, sizing, and insurance chapters of this issue apply unchanged.
The short of it: sapphire for tradition without the diamond invoice, moissanite for fire and frugality, and soft stones only for people who enjoy upkeep.