Olive oil may seem simple. The olive is a fruit. Its oil is its juice. Virgin oil is mechanically pressed without using heat or chemical processes. Extra virgin oil has particularly low acidity. You use it for cooking and salads.

But seriously, any gourmet worth his hand-harvested French sea salt also needs to know that a "peppery" Tuscan-style olive oil pairs well with arugula and radicchio, that butter lettuce and micro greens should be dressed with a less "assertive" oil—hold the vinegar—and that a hint of bananas marks the flavor of one oil produced in Alpes de Haute-Provence. Olive oil is a deep subject for aficionados who characterize varieties using a vernacular familiar to wine connoisseurs. The passion is growing in Napa Valley, California, where small producers make exquisite boutique oils and a tour operation caters to enthusiasts (www.greatolivetours.com).

Rose Malindretos, who spent summers on Crete surrounded by olive groves and now represents the O & Co. shops that specialize in olive oil, says that more than a hundred different olive varieties can be pressed for oil. Some varieties are specific to regions. Climate and soil govern taste, which varies from year to year. The parent company, Oliviers and Co. (www.oliviersandco.com), currently cellars 33 oils from 11 Mediterranean countries. Some producers bottle but a few hundred liters a year. Such oils are intended as condiments. Peppery oil from the frantoio olive is ideal for drizzling over a steak or roast. The tanche olive yields a buttery oil, perfect for dressing greens, poached fish or even strawberries.

Many olive oil millers combine varieties to their taste, to make oil specific to an estate. Rather than mixing oils, they press several fruit varieties together. Californians Rachel and William J. Casey take a slightly different approach at Poplar Hill (www.poplarhilloliveoil.com), their St. Helena estate: half the hand-picked fruit from the estate's 40 acres of 88 lucca olive trees are pressed when new and green for pungency; the other half when ripe and black for mellowness. The product: less than 500 vintage bottles of delicate, elegant oil, recommended for "bumping up" the flavor of fresh micro greens and as a substitute for butter in baking.

—Warren Kalbacker