By Ron B. Davis Jr., Georgetown University
What produces the colors we find in precious stones are trace elements present in such small quantities that they may not even be listed in a mineral’s simplified formula. Precious stones such as ruby, sapphire, and turquoise owe their famously vibrant colors to trace amounts of some other type of metal, located along the top row of the d-block.
The Different Hues
Different ions of iron give different colors to a variety of gemstones, including amethyst, aquamarine, garnet, and topaz. Emerald, on the other hand, gets its green from vanadium. The famous blue hue of sapphire comes when titanium and iron take up residence in aluminum oxides. Ruby’s red happens when chromium replaces some aluminum ions in ordinary aluminum oxide. Jade gets its color from a mixture of chromium and iron.
Spinel is an ordinary oxide of magnesium and aluminum, but colors ‘pop’ when chromium, iron, cobalt, or nickel get involved. Turquoise, however, gets its distinctive color from the presence of copper in the crystal.
Thus, closer inspection reveals that many of the most well-known precious stones seem to owe their prized color to the d-block transition metals, 22 through 29, all of which group neatly into the top row of the periodic table’s d-block.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Specific Wavelengths of Light Absorbed
Electrons in the d-subshells of transition metals can absorb energy from light, promoting them from one d-orbital to another of slightly different energy. And it so happens that in the 3-d subshell in particular, these absorbed energies often coincide with one or another photon of visible light.
Hence, white light that shines on minerals containing ions of these special elements is not all reflected. Specific wavelengths of that light are absorbed, with the remainder creating the appearance of color, sometimes to stunning and beautiful effect.
Scandium and Yttrium
The transition metals play indispensable roles in material science, biology, medicine, chemical analysis, and so many more fields. And, as one might expect, transition metals with similar properties, and therefore similarly useful applications, often tend to cluster together on the periodic table.
The first two elements that we come upon on the d-block are scandium and yttrium. Both are named, the first broadly and the second more precisely, for the location where they were first discovered, in the town of Ytterby, Sweden, on the Scandinavian peninsula. It was there that a strange, heavy black mineral was first discovered.
This mineral proved to contain a number of undiscovered elements, these two from the d block, but also many from the f-block of the table.
The First D-block Element
In 1869, Mendeleev correctly predicted the discovery, which happened ten years later, of an element between calcium and titanium. However, his lack of distinct groups for most of the metals made him incorrectly refer to this prediction as eka-boron. Scandium actually is the very first d-block element, and the first in its own column, group 3 of the modern table.
Scandium is a soft, silver-white metal that tarnishes in air, not unlike it’s s-block neighbors, calcium and potassium. Both scandium and yttrium are only a little less abundant in the Earth’s crust than copper or nickel. But these elements are more often used in small amounts to fine-tune the properties of other elements.
Lightweight Alloys and High-power Lasers
A pinch of scandium added to aluminum makes a very strong, lightweight alloy, valuable in jet fighters and high-end bicycles. Adding a bit of yttrium to aluminum plus chromium can make an alloy with unusual heat resistance.
Yttrium added to aluminum plus garnet-type silicate minerals—a combination called YAG—makes a very hard, diamond-like gemstone that’s used in high-power lasers.
Just a little bit goes a long way for both these elements. Globally, just a few dozen to a few hundred tons of each are produced annually.
Low-density Metals with Good Conductivity
Scandium and yttrium are relatively low-density metals with good electrical conductivity, just as one might expect for an element on this left side of the block.
The most common oxide for each is Sc2O3 and Y2O3. Based on this ratio of elements, it is clear that the 3+ ion of these III-B metals is the most stable, just like the group III-A metals aluminum, gallium, indium, and thallium.
But what sets scandium and yttrium apart from those elements is that scandium and yttrium become a 3+ ion by losing electrons from a different subshell. Aluminum for example, is a 3s2-3p1 metal, scandium is a 4s2-3d1 metal. Although each needs to lose three electrons to achieve a noble-gas configuration, those electrons come from different subshells.
The same relation we saw for scandium and yttrium holds true for the transition metals zirconium and hafnium in the next group over.
Zirconium and Hafnium
Both zirconium and hafnium have a single common oxidation state that is achieved by loss of all of their outermost d- and s- electrons. Loss of 4 electrons leaves each with a noble-gas configuration. Their most common oxides are then ZrO2 and HfO2, making them behave, in that respect, much like the group IV A metals tin, and lead.
In this cluster of the d-block we find many metals that are famous for their toughness and durability. Titanium, vanadium, and chromium all appear in this region of the d-block.
Thus, if we look at a projection of the elements’ hardness on the table, we can see that there is something special about these early transition metals. What’s special is that their d-subshell electrons are unpaired, spread out through their d-orbitals. Unpaired electrons can join very effectively in metallic bonding.
In addition, we see an increase in melting point through this region of the table. And for essentially the same reason, we also see a general increase in hardness, with some famously hard materials that are only slightly softer than diamond!
Common Questions about D-Block Transition Metals
Most well-known precious stones seem to owe their prized color to the d-block transition metals, 22 through 29, all of which group neatly into the top row of the periodic table’s d-block.
Yttrium added to aluminum plus garnet-type silicate minerals—a combination called YAG—makes a very hard, diamond-like gemstone that’s used in high-power lasers.
In the cluster of the d-block we find many metals that are famous for their toughness and durability. Titanium, vanadium, and chromium all appear in this region of the d-block.
