Trichomes are what produce the THC and the many other cannabinoids that are unique to the cannabis plant.
On a live plant, they sparkle under light.
As the marijuana plant matures, the bulbs at the top of the trichomes swell and become resinous and sticky.
Once the cannabis plant is harvested, it is dried and cured, causing the head to harden and turn amber as it begins to degrade.
For an historical perspective on Trichomes, visit The Little Green Schoolhouse comic at Dankdoodles.com by clicking the image below.