Is this sapphire heated or untreated?

The listing does not state treatment. Ask the seller for disclosure before buying, and request an independent lab report if untreated matters to you.

What size setting will a 7.75 x 7.7 mm stone fit?

An octagon stone this size needs a custom-cut head or a setting made for its exact dimensions. A standard round or oval mount will not seat it properly.

Can I order it with a different gemstone?

Yes. The seller says the design can be made with gemstones of your choosing, and asks that you contact them before purchase to arrange a custom stone.

Does it come with a ring?

No. This is a loose sapphire only. The metal, band and ring size are chosen separately when you have it set.

Why does the stone show two colors?

Bi-color sapphires grow with color zoning, so blue and yellow sit in bands within the same crystal. The octagon cut lines those zones up across the table.

How should I care for an unset sapphire?

Keep it in a soft pouch or its own box so the facet edges do not chip against harder stones. Clean with warm water and mild soap, not ultrasonic cleaners.