Rocks as Gems

The word “rock” doesn’t normally conjure up images of beautiful jewelry, but as I explained in my very first blog post (, some rocks are also considered gems. While many minerals meet the criteria of beauty, rarity, and durability to be considered gems, very few rocks do.

Because the conditions under which rocks form can vary greatly, there can be very different looking results. Even within a specific type of rock, although the main mineral components are the same, trace elements or other minerals, sizes of individual grains, relative amounts of each mineral, and more, may differ. Causes for these differences include: the elements available in the melt source, temperature, and pressure. Most of the rocks used as gems have individual minerals that are visible to the naked eye, but some, such as basalt (sold as “lava”) have mineral grains too small to see. This variability is part of the beauty of the rock, but can also make it very difficult to match pieces for jewelry.

Enjoy these examples of rocks that are gems – I’ve listed the main constituent minerals.

Lapis lazuli

Blue: lazurite
Brassy/gold: pyrite
White: calcite

Ruby in zoisite

Green: zoisite
Red: ruby
Black: hornblende

See my blog:

Tiger Iron

Gray: hematite
Gold: tigers eye
Red: jasper

See my blog on hematite:


Green: epidote
Pink/orange: potassium feldspar
Gray: quartz

Lava (basalt)

Individual grains too small to see but include:
Plagioclase feldspar
Pyroxene minerals
and many other accessory minerals

Graphic Granite

(labeled as “feldspar” when I purchased it)
Gray: quartz
White: feldspar
and many other accessory minerals

So yes, we geologists love rocks, but now even non-geologists have a reason to enjoy them too!

Ruby in Zoisite

This month I’m going to tell you about an interesting but not particularly common gemstone – ruby in zoisite.

Ruby in zoisite was discovered in 1954 in Tanzania, Africa, and so far, this is the only location it has been found. Other names for this gem are “anyolite” (from the Masai word “anyoli,” meaning green) and “Tanganyika artstone.”

Ruby in zoisite is a rock that is comprised of 3 minerals: corundum (red), zoisite (green), and hornblende (black). The colors can be so bright that it is hard to believe it’s a natural stone. The rough stone can be difficult to work with because of the differences in hardness of the constituent minerals. Ruby (corundum) has a hardness of 9 on the Mohs scale of mineral hardness, while zoisite is 6-7 and hornblende is 5-6. Ruby in fuchsite looks similar to ruby in zoisite, but it lacks the black hornblende. In addition, fuchsite is a mica and is very soft (2-3 on the Mohs scale) which makes it less desirable for use in jewelry.

Ruby in zoisite for sale in Tucson
(c) Mochi’s Gifts, 2019

Zoisite comes in a variety of colors including colorless, violet-blue, grey, yellow, brown, pink, and green. The violet-blue colored zoisite is better known as Tanzanite, also only found in Tanzania, Africa. Another zoisite gem you may have heard of is thulite, the opaque, pink variety.

Ruby from this source is opaque and not good for cut gems. Ruby crystals tend to be very dark pink to red and often show the hexagonal outline of the crystal. The ruby inclusions can vary greatly in size from just small specks of red to crystals several inches across (see photo above).

Ruby in zoisite necklace.
(c) Mochi’s Gifts, 2019

Ruby in zoisite is often used in carvings that make use of the arrangement of the colorful minerals. The color combination in ruby in zoisite isn’t for everyone, but if you like it, it makes for very striking and unique pieces of jewelry.