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Dwarves' Earth Treasures Online Museum:
What are the Datolites?
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    At any rock shows or rock shops, we can expect to encounter the datolites in their typical forms: colorless crystals with greenish hue, but there is one and only place in the world where a very unusual type of datolites is found that can make other datolites from other locations around the world seem dull. Those are the datolite nodules of Michigan coming in a full(?) spectrum of colors.

 The name "Datolite" is derived from "dateisthai" which is Greek for "to divide" referring to how datolite crystal aggregates tend to easily crumble.
  Datolite's composition is hydrous calcium borosilicate (CaBSiO4(OH)) meaning that each molecule contain calcium and boron atoms combined with silica "pyramid" and a water molecule that is missing a hydrogen atom.
 It appears that the datolites are formed from the deposition of boron-rich silica hydrothermal (hot water) solutions filling in the fissures, seams & cavities within the basalt lava rocks.

Identification keys:
    Location and type of rock host are the main keys to the identification of datolite for they can make identification much easier. The datolites are always found in well-weathered basalt lava rocks, and rarely in other igneous rocks. The examples of known locations:  September Mine of Russia, Westfield, Mass., West Paterson, New Jersey, and copper mines of Michigan.
    Datolite are typically found with calcite, quartz, fluorapophyllite, prehnite, zeolites, and other minerals associated with basalt lava rocks.
    The datolite crystals tend to be wedge shaped with typical faint greenish hue. They have no cleavage meaning that when they break, they doesn't follow their crystal structure, resulting in irregular breakage. Calcite will break along its crystal structure producing flat and reflective cleavage while pointy quartz crystals break like glass. Similar colored analcime crystals are more rounded (many faces) just like garnets.
    The porcelain nodules of Michigan can be confused with howlite, another boron mineral, but the howlite nodules are typically found in the ancient & dried sea/lake beds of southern California. Howlite is much softer and larger than datolite, and has gray surface with white color inside. Any datolite nodules bigger than baseballs are rare.

    Here's an interesting note about the datolites of Michigan: Anywhere else, datolite crystals are abundant while the nodules are almost nonexistent, and yet in Michigan, the nodules are abundant while the crystals are rare. It seems that there was something in the basalt lava beds that influenced how the datolites were formed. The Copper Falls mine, and adjust fissures of Keweenaw County, MI are the only known places where Michigan datolite crystals are somewhat abundant, and some are pink due to copper inclusions.

Datolites of Michigan:
    Datolites of Michigan are much more appreciated than any other datolite species from other locations, and that is because the datolite nodules of Michigan come in so many colors from red to black. To make them even more desirable is that they can be cut and polished into colorful cabs.
    The datolite nodules tend to look like cauliflowers, balls or both, but since datolites have formed late in the mineral-forming processes, they commonly fill in the cavities/veins or enclosing other minerals such as copper crystals. See the picture to see what datolites in their raw forms look like.

    It is common for datolite nodules to be found in the mud-filled veins and fissures in highly decomposed/brecciated basalt lava lodes. I remember watching Richard Whiteman, owner of Caledonia Mine, digging datolite nodules out of a mud-filled seam with just his hands. Some datolite nodules are found in solid basalt rocks separated by only thin film of chloride, copper or other minerals.
    Since the basalt rocks of Michigan were mined mainly for copper and silver, datolites and other minerals were dumped in the "poor" rock piles. The rock piles had been searched for decades so it would take a plenty of luck, digging and good eye to spot any datolites in the rock piles. It's easy to find dirty cauliflower-like nodules, but it's easy to overlook ball-like ones or even those almost hidden inside basalt rocks. I once searched Delaware mine several times and found 35 small datolite "marbles", but almost no cauliflower nodules due to the fact that local people already found them before I did. The datolites from the Delaware, and Caledonia Mines are most commonly encountered, and they still can be found there.

    How did the datolite nodules got their color? They get their colors from the aggregates of colored mineral inclusions which can be seen as colorful dots (balls if thinking in 3D) on some datolite specimens. More the "dots" a datolite nodule has, stronger its color (see Rainbow of Datolites for examples).
    White is most common color among the datolites due to either lack of mineral inclusions, bleaching by water or both. It is also common for colored datolites to be white rimmed due to bleaching by water.
    The tightly packed aggregates of copper mineral, chalcotrichite provides strong red color as in case of well-known pink & red datolites from Delaware mine.
    Chalcotrichite also provided the orange and salmon colors, and the difference from the red aggregates is that the aggregates are larger and more orangish. That is why orange "dots" tend to be easier to be seen than those of red "dots".
    How the datolites got their yellow colors are not well understood. It was said that the yellow color was caused by iron replacing a calcium atom, which would explain why we never see any yellow "dots" in such nodules. More iron replacing the calcium, stronger the yellow color.
    The green and blue colors of datolites got their colors from a mineral paratacamite that were formed by reactions between native copper inclusions and ancient salt brines. Malachite and azurite may also be responsible for some green colors.
    Some datolites especially those from the shores of Keweenaw Pennisula and Isle Royale are opaque with greenish hue just like the datolites crystals from other places outside Michigan. Since they have been found underwater, water bleaches the surface and the natural cracks within datolites giving them chambered look.
    There were reports of datolite with purple colors, but I haven't seen any so far. I have one that looked bit purplish, but I'm not sure whether it's more grayish or purplish. Our eyes can be quite deceiving.
    There are also some reports of black datolites. I have seen several so far and the black color may be due to manganese or silver inclusions.
    Some datolite nodules are known to variegated, meaning several colors in one nodule. Best example would be the datolites of Mesnard and Quincy mines which each datolite contains a mix of strong yellow, red and orange colors. Some datolites have color zones that demonstrate how a datolite nodule grew in sequences.
    Many datolite nodules contain copper inclusions, but they rarely provide any colors except gray "spots", but there are some cases which the copper inclusions are so fine that they provide brownish-red colors as in case of the brownish-red datolites from Caledonia Mine.  Some datolites from Centennial and Ojibway mines showed the zoning of copper inclusions which can show how the datolites grew.

Conclusion:
    Datolite always come in colorless wedgelike crystals with greenish hue and no cleavage, and are found in any cavities of the well-weathered basalt lava rocks. Only one special exception is that datolites from Michigan tend to come in cauliflowerlike nodules which are harder and more colorful than howlite nodules. The datolites of Michigan got their colors from the aggregates of colorful copper minerals with some bleaching by water.  Compared to other datolites around the world, datolite nodules of Michigan are freaks, and that's what them so fun to collect!