ruby as a rough specimen in matrix with one reasonably large 7mm x 3mm hexagonal crystal showing through the matrix and several smaller pieces of ruby crystal showing through as well
This is the red variety of corundum. It is the best known and most prized of this hard and durable mineral.
from Wikipedia –
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. It is considered one of the four precious stones, together with sapphire, emerald and diamond.