Moonstone Vs Opal: What's the Difference?

Both Moonstone and Opal are stunning gemstones with ethereal beauty, but they have distinct differences in terms of appearance, composition, durability, and value. Here's a breakdown:

Definition

Moonstone is a sodium potassium aluminium silicate with the chemical formula (Na,K)AlSi₃O₈ and belongs to the feldspar group.

Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%.

Formation

Moonstone: A feldspar mineral, typically composed of orthoclase and albite. Forms in igneous and metamorphic rocks.

Opal: A hydrated amorphous form of silica (silicon dioxide with water). Forms in sedimentary rocks, volcanic cavities, and hot springs.

Composition

Moonstone is composed of two feldspar species, orthoclase and albite. The two species are intermingled. Then, as the newly formed mineral cools, the intergrowth of orthoclase and albite separates into stacked, alternating layers.

Opal is a hydrated amorphous form of silica (SiO₂·nH₂O), It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.

Moonstone Vs Opal

Appearance

Moonstone: Usually white, grey, or peach with a pearly sheen called adularescence that shimmers like moonlight. The adularescence can show flashes of blue, green, or gold depending on the viewing angle.

Opal: Found in a vast array of colors, including white, black, blue, green, red, and even rainbow flashes. Some varieties like fire opal or black opal exhibit a play of color called opalescence, a different phenomenon than moonstone's adularescence.

Cause of Colors

Moonstone is a gem from the feldspar group and is composed of layers of albite – potassium rich aluminium silicate and orthoclase – sodium rich aluminum silicate. The schiller is produced because of light interference caused by the light having to weave its way through the layers with slightly different optical properties. The adularescence is caused by scattered light passing through the exsolution lamellae that act as scattering centres, creating a bluish hue/ lustre.

Opal - is opal which exhibits the phenomenon known as play of colour, which is produced by the diffraction of white light through a micro-structure of orderly arrayed spheres of silica.The internal structure of precious opal makes it diffract light; depending on the conditions in which it formed, it can take on many colors.

Types

Moonstones come in a variety of colors. The body color can range from colorless to gray, brown, yellow, green, or pink. The clarity ranges from transparent to translucent.

There are many different types of natural opals. There is black opal, white opal, crystal opal and jelly opal. There are other kinds - fire opal, hyalite, water opal, hydrophane and honey opal.

Durability

Moonstone: Slightly harder than opal, with a Mohs hardness of 6-6.5. Requires some caution against scratches and breakage, but suitable for daily wear with care.

Opal: Softer than moonstone, with a Mohs hardness of 5.5-6.5 More susceptible to scratches and chips, especially delicate varieties. Can crack with sudden temperature changes.

Value

Moonstone: Generally less expensive than opal, with high-quality specimens reaching a few hundred dollars per carat. Common varieties can be found for much less.

Opal: Can range significantly in price depending on variety, color, play of color, and clarity. Common opals may be quite affordable, while rare gem-quality specimens, like black opal, can fetch thousands or even millions per carat.