The blue oak earns its common name from the tint of its exquisite deciduous foliage, particularly when the beautiful new growth emerges in early spring. The tree can reach 20-60 feet tall, growing in thin soils or from crevices between rocks, and it is an important member of many old-growth oak groves in the east bay and elsewhere.
Note that photos are Quercus kelogii, Black Oak with similar spring flush but more deeply cut lobes and douglasii turns blue and leathery in summer.