Katchatheevu was geologically linked to Indian land mass:

The Centre has taken the controversial stand that Katchatheevu in the Palk Strait off Rameswaram coast in Tamil Nadu was never part of India, but a senior geologist and remote sensing expert says remote sensing satellite data shows that the islet was geologically linked to Indian land mass.
"Katchatheevu belonged to the Ramanathapuram Samasthanam (kingdom) not only as per revenue record, but even geologically it was connected to Indian land mass," said Prof S M Ramasamy, Director of Centre for Remote Sensing in the Bharathidasan University here.
The Indian Remote Sensing (IRS) satellite data and ETOPO (sea bed topography) data proved that geologically, Katchatheevu had continuity to Pisasumunai at the tip of Rameswaram island, he told PTI.

While islands off Jaffna in Sri Lanka upto Delft island were accurately arranged encircling the northern province and also had semi-consolidated Holocene sediments of more than 4,000 years, Katchatheevu is made of sands of later period equivalent to Rameswaram which is around 3,500 years old as revealed in radio carbon dating.
In its response to petitions by Chief Minister Jayalalithaa and others in Supreme Court seeking retrieval of Katchatheevu, the Union Government has taken the stand that no territory belonging to India was ceded to Sri Lanka, implying the islet was never part of the country.