Larimar is one of the rarest stones being mined today.  As such, good examples tend to be relatively expensive, and fake larimar is everywhere!  Here is what it IS:

Larimar is a form of pectolite (with copper) found only in a single place in the Dominican Republic. It is an opaque sky blue stone with white streaks occuring in patterns reminiscent of the ocean's surface. It's transluscent, almost glowing in direct sunlight due to slight refraction in the play of light. There are often some red to brown impurities, which significantly decrease the value of the stone.

Larimar is hand-mined with picks and shovels because it occurs in narrow veins.  Due to its brittleness, Larimar is usually shaped and polished (but not faceted), much like turquoise. Larimar has a hardness of 4.5 - 6.0 and a specific gravity of 2.7 to 2.9.

Larimar stones are usually polished irregular shapes, but large finished shapes are sculpted, usually at significant cost.

Larimar was named for Larisa (the daughter of Miguel Mendez, a  geologist who helped reintroduce this stone) and mar (the Spanish word for sea).  The stone was introduced to the international market in the early 70's, and became heavily in demand in the last several years due to its remarkable color, and popularity among celebrities including Tyra Banks, Cameron Diaz, and Katherine Heigl. 

Here is what it ISN'T:

Larimar isn't a color, or a form of Lace Agate, or a form of any other stone but Pectolite. 

Larimar is not enhanced in any way, meaning that there should not be dye present, stabilizers applied, or laquer finish applied. 

Larimar isn't a gem, but a semi-prescious stone. 

Larimar isn't any color but blue, as all other colors of Pectolite are available in many other mines in the world, and are not hand mined or expensive.

Enjoy Larimar as you would any fine stone, avoiding contact with chemicals like harsh cleaners, hairspray, chlorine, oily products like massage lotions.  Clean with a jewelry cloth, or a very delicate shampoo and water mixture.  Rinse thoroghly.