Granville Austin, a renowned constitutional historian, famously described the Indian Constitution as the 'cornerstone of a nation,' emphasizing its foundational role in shaping India's identity, democratic values, and socio-economic transformation. This assertion highlights that the Constitution is not merely a legal document but a comprehensive social and political charter that has guided India's journey as a sovereign, democratic republic.

Analysis of Austin's assertion:

1. Foundational Principles: The Preamble itself lays down the core ideals of Justice, Liberty, Equality, and Fraternity, establishing the moral and ethical framework for the nation. It declares India a Sovereign, Socialist, Secular, Democratic Republic, defining its fundamental character.
2. Fundamental Rights and Directive Principles: Austin famously called the Fundamental Rights (FRs) and Directive Principles of State Policy (DPSPs) the 'conscience of the Constitution.' FRs guarantee individual liberties and equality, forming the bedrock of citizenship and protecting citizens from state overreach. DPSPs, though non-justiciable, lay down the socio-economic goals and aspirations, guiding the state towards a welfare society and ensuring social justice. Together, they provide a blueprint for a just and equitable society.
3. Democratic Framework: The Constitution established a robust parliamentary democratic system, ensuring representative government accountable to the people. It provided for universal adult franchise, a revolutionary step for a newly independent nation, empowering every citizen regardless of caste, creed, or gender.
4. Federal Structure with Unitary Bias: For a nation as diverse as India, the Constitution crafted a unique federal structure that balances regional autonomy with a strong unitary bias, crucial for maintaining national unity and integrity. This framework has successfully accommodated linguistic, cultural, and religious diversity while preventing fragmentation.
5. Independent Judiciary: The establishment of an independent and powerful judiciary, with the Supreme Court at its apex, serves as the guardian of the Constitution and protector of fundamental rights. It ensures the rule of law and acts as a check on the powers of the executive and legislature.
6. Nation-Building Instrument: The Constitution provided the necessary framework for integrating diverse princely states, linguistic groups, and communities into a unified nation. It facilitated social reforms, such as the abolition of untouchability (Article 17), and laid the groundwork for affirmative action policies, crucial for addressing historical injustices.
7. Adaptability and Resilience: The amending power (Article 368) allows the Constitution to adapt to changing times and societal needs while preserving its basic structure, demonstrating its resilience and foresight.

In conclusion, Austin's characterization is apt because the Indian Constitution has indeed served as the fundamental pillar upon which the nation has been built. It has provided stability, guided development, fostered democratic values, and ensured the protection of rights, making it the indispensable 'cornerstone' of the Indian nation.