Green and greenish rocks get their color from green minerals, but a lot depends on the type of rock: igneous rocks, sedimentary rocks and metamorphic rocks each have their own set of green minerals. It will help if you know how to distinguish those major rock classes as well as "How to Look at a Rock." But first, be sure you're looking at a fresh surface! Don't let a coat of green algae fool you. Green minerals can be hard to identify until you've learned something about the most common ones. Here they are, along with their usual luster and hardness. If your green or greenish mineral doesn't fit one of these, there are many more possibilities. Another tidbit: green colors usually arise from the presence of iron or chromium and sometimes manganese.
A shiny medium-green mineral with many long, thin crystals is likely to be actinolite. Look for it exclusively in metamorphic rocks, where it forms crystals in marble or is disseminated in greenstone. Its color is from iron; the white variety without iron is tremolite. Luster glassy to pearly; hardness 56.
It's the most widespread green mineral, but one you may never see by itself. In microscopic form, chlorite gives a dull olive-green color to a wide range of metamorphic rocks from slate and phyllite to schist. It is also found in small spots or masses in which it displays a flaky structure like that of a mica mineral, but it gleams rather than sparkles and doesn't split into flexible sheets. Luster pearly; hardness 22½.
Epidote is common in medium-grade metamorphic rocks as well as late-stage igneous rocks such as pegmatites. It's typically a pistachio or avocado green when it occurs in the massive formcrystals have a wider color range. Luster dull to pearly; hardness 67.
Ron Schott (Flickr CC BY-NC-SA 2.0)
This is the usual green mineral found in greenish marine sandstones and the gardening amendment known as greensand. It's a mica mineral, but because it forms by alteration of other micas it never makes crystals. You'll generally see it in the form of a blue-green color rather than a separate mineral. Luster dull; hardness 2.
Few minerals excite the rockhound like jade, but it can be hard to distinguish. Two minerals, jadeite and nephrite, are recognized as true jade. Both occur where serpentinite is found but form at higher pressures and temperatures. Nephrite (a microcrystalline form of actinolite) has a hardness of 56; jadeite (a sodium pyroxene mineral) has a hardness of 6½7.
Dark primary igneous rocks (basalt, gabbro and so on) are the exclusive home of olivine. It's usually found in small clear olive-green grains and stubby crystals. A rock made entirely of olivine is called dunite. Olivine breaks down at the Earth's surface by chemical weathering. It would rather live deep beneath the Earth's crust, where it is most stable. Olivine gives the rock peridotite its name, peridot being the gem variety of olivine. Luster glassy; hardness 6½7.
fluor_doublet (Flickr CC BY-NC-SA 2.0)
Rocks metamorphosed by hot-water solutions may have prehnite crusts and botryoidal clusters along with pockets of zeolite minerals. Prehnite has a light "bottle-green" color and is quite translucent. Any rock shop will have prehnite specimens you can learn this mineral from. Luster glassy; hardness 66½.
Serpentine is a metamorphic mineral that occurs in some marbles but more often keeps to itself in the distinctive rock serpentinite. It typically occurs in shiny, mottled, streamlined shapes and never in crystals (except sometimes as asbestos fibers). Its color ranges from white to black but is mostly dark olive-green. Serpentine is a sign of high-magnesium (typically deep-sea) lavas that have been thoroughly altered by hydrothermal activity. Luster greasy; hardness 25.