A gemstone is a mineral, with applications in many aspects of life on Roshar. On Earth, some are the same type of crystal but with different color and chemical signatures. Sapphire and ruby are both types of corundum. Smokestone and amethyst are both types of quartz. Emerald and heliodor are both types of beryl. Zircon is itself a mineral that manifests in many colors; blue is enhanced by heat. Topaz is a rare silicate mineral that also manifests in a variety of colors. Diamond too is a rare mineral. Garnet is comprised of a group of silicate minerals.
Though mining efforts do indeed result in the acquisition of gemstones on Roshar, the harvesting of gemhearts - which grow within greatshells - produces another significant source. Larger species of greatshell, like chasmfiends, produce gemhearts of great size, which are incredibly useful in Soulcasting as larger stones are less likely to break from the strain. This trait has led to many species of greatshells, such as the lanceryn.
Gemstones are used in spheres, which are the primary currency of Roshar. Their ability to absorb Stormlight (infused) make them a source of magical potency necessary for certain types of magic, such as Soulcasting, powering fabrials replicating types of Surgebinding, and as common sources of illumination. An infused gemstone can also trap a spren. The type of spren that can be trapped depends on the type of gemstone and its cut. This is also the basis for fabrials.
The type of gemstone and the size greatly influence the efficiency of the fabrial; the larger the stone, the more Stormlight it can gather and maintain its effect. Stormlight infused gemstones glow more brightly when properly cut. Gemstones are differentiated by color. The color signature needs to be of a specific variety to provide the proper key to accessing the power of transformation.
Only those gemstones used in currency are called by nicknames. The "mark" is the intermediate size of gemstone (between "chip" and "broam"); marks and chips are sometimes referred to by color rather than gem: "skymark" (sapphire mark), "clearchip" (diamond chip), "firemark" (ruby mark), and "bloodmark" (garnet mark) are those referenced most. There doesn't seem to be any similar nickname for emerald, though, which is odd because they're the most valuable.
Each Polestone is associated with one of the Ten Numbers, one of the Ten Essences, a Body Focus, one of the Soulcasting Properties, and Primary and Secondary Divine Attributes in traditional Vorin symbolism. Their colors are present on the more complex representation of the divine prism that has two sets of ten spheres:
- Associated with the Number Jes, the Essence Zephyr, Inhalation, and the Attributes Protecting/Leading.
- Used to Soulcast translucent gases, like air.
- Sapphire spheres are given the "sky-" prefix.
- Associated with the Number Nan, the Essence Vapor, Exhalation, and the Attributes Learned/Giving.
- Used to Soulcast opaque gases, like smoke and fog.
- One of the gems in Jasnah's Soulcaster. It went dark and cracked when she used it to dissolve stone and again when she attacked a group of footpads.
- Used in a clock fabrial belonging to Lirin.
- Mraize possesses an attracting fabrial, powered by a smokestone.
- Associated with the Number Chach, the Essence Spark, The Soul, and the Attributes Brave/Obedient.
- Used to Soulcast fire.
- Ruby spheres are given the "fire-" prefix.
- One of the gems in Jasnah's Soulcaster.
- Used in the Diminisher fabrial Navani had to remove pain.
- Used in the Augmenter fabrial to give off heat.
- Used in Paired Conjoiner fabrials like spanreeds.
- Associated with the the Number Vev, the Essence Lucentia, The Eyes, and the Attributes Loving/Healing. (The eyes are associated with crystal and glass because they are the windows into the mind and the spirit.)
- Used to Soulcast crystals, like quartz, and glass.
- Diamond spheres are given the "clear-" prefix.
- One of the gems in Jasnah's Soulcaster. When Jasnah turned a footpad to crystal, the diamond in her Soulcaster faded.
- Teft discovered that Kaladin could use Stormlight when he placed three diamond spheres in Kaladin's hand while he was recovering from injuries sustained while being strung up in a highstorm. Kaladin was healed slightly and the spheres became dim.
- Infused diamond spheres are often used in fabrials for lighting, casting a bright white light.
- Associated with the Number Palah, the Essence Pulp, The Hair, and the Attributes Just/Confident.
- Used to Soulcast plant material, like wood and moss.
- Emeralds can be used to capture flamespren.
- A very large emerald gemheart was found by King Elhokar when a chasmfiend was killed.
- Associated with the Number Shash, the Essence Blood, The Blood, and the Attributes Creative/Honest.
- Used to Soulcast the eight types of blood and all other non-oil liquids.
- Garnet spheres are given the "blood-" prefix.
- Used in a type of water attracting fabrial.
- Jasnah used a garnet to Soulcast poison from Shallan. 
- Associated with the Number Betab, the Essence Tallow, Oil, and the Attributes Wise/Careful.
- Used to Soulcast oils.
- Infused zircon gives a pale blue light.
- Associated with the Number Kak, the Essence Foil, The Nails, and the Attributes Resolute/Builder.
- Used to Soulcast metals.
- Infused amethyst gives a soft violet light.
- Used in Paired Reverser Fabrials to create opposite reactions.
- Associated with the Number Tanat, the Essence Talus, The Bone, and the Attributes Dependable/Resourceful.
- Used to Soulcast rock and stone.
- In one of Dalinar's visions ... "A woman in delicate shardplate" used a bright topaz entwined with heliodor in a fine metal framework - a fabrial replicating Regrowth, an application of the Surge of Progression - to heal him and remove the pain.
- Associated with the Number Ishi, the Essence Sinew, Flesh, and the Attributes Pious/Guiding.
- Used to Soulcast animal material, like meat and flesh.
- Used in Warning Fabrials.
- See topaz above for healing by a Knight Radiant.
- Vstim's fabrial is powered by a large heliodor.
Ico tells Kaladin that in Celebrant, the moneychangers have perfect gemstones that can hold Stormlight indefinitely, and that Light in a perfect stone doesn't run out, so one can give Light to moneychangers, who use devices to transfer it from smaller gemstones to their perfect ones. They then give one credit to spend in the city.
Kaladin thinks about this and wonders if such a thing exists on his side (i.e., the Physical Realm). If there really were flawless stones that could hold Stormlight without ever running out, that seems to him to be important to know. It could mean the difference between life and death for Radiants during the Weeping.
- Sapphire and Ruby
- The Way of Kings, 13. Ten Heartbeats
- The Way of Kings, 36. The Lesson
- The Way of Kings, 3. City of Bells
- Navani's Notebook
- The Way of Kings, 27. Chasm Duty
- Power of transformation ...
- The Way of Kings: Ars Arcanum
- The Way of Kings, 33. Cymatics
- The Way of Kings, 8. Nearer the Flame
- Words of Radiance, 43. The Ghostbloods
- The Way of Kings, 7. Anything Reasonable
- The Way of Kings, 11. Droplets
- The Way of Kings, 46. Child of Tanavast
- Words of Radiance, 86. Patterns Of Light
- Navani's Notebook
- The Way of Kings, 17. A Bloody, Red Sunset
- Words of Radiance, 82. For Glory Lit
- The Way of Kings, 48. Strawberry
- The Way of Kings, 37. Sides
- The Way of Kings, 19. Starfalls
- Oathbringer, 99. Reachers